Category Archives: The Crime Scene
At a recent lecture I took a question from the audience regarding casting bloody impressions (footwear and fingerprints) on human skin so I thought this might be an interesting topic to bring to your attention. Bloody prints can be challenging to crime scene investigators, especially on dark colored surfaces. Most CSIs will photograph the prints (maybe IR) or try to enhance them with a blood reagent like Leuco-crystal Violet (LCV) or Luminol. The goal is to lift or transfer the impression onto a lighter material so the unique details can be easily photographed.
I’ve written about casting footwear impressions in previous posts but I don’t think I’ve ever written about casting fingerprint impressions. Regardless, the method is basically the same. I’ve actually done research with bloodstained evidence using various casting materials on surfaces such as concrete, fabric, and human skin. One of the casting materials that reproduced the greatest clarity of detail in the impressions is something you’ve probably had in your mouth; Alginate. Alginate is a common casting material used in dental offices for casting your upper and lower teeth and gums. Dentists put it in trays that you bite down on.
Alginate comes in powder form and is mixed 1:1 with water to make a pancake like batter. The material is then poured/spread over the dried impression and allowed to set for about thirty minutes. Alginate can even be used to lift latent impressions developed with color staining reagents like LCV or LMG. The downside to Alginate is that as it dries it becomes very brittle and shape distorted. That means that your character has to photograph the impression immediately after the cast is lifted from the surface. As you would expect, this material doesn’t work well on hairy areas of skin but areas like ankles, wrists, necks, etc can produce excellent results. You can also use this material to lift fingerprints developed with powders but there are other methods that are better suited for such things. I’ll male a note to do a posting on that at some point in the near future.
So if you have bloody shoe, foot, or finger impressions at your crime scene don’t fret! Your characters do have an option to collect such evidence, especially on dark surfaces like a black leather jacket or multicolored bed sheets. If you have an inexperienced CSI character you could introduce a little tension by letting them forget to photograph the cast and within a few hours they have to work with a warped/distorted and brittle cast.
There is an age old tradition in some cultures to fire celebratory shots into the air on certain anniversaries, events (weddings), or accidents. In the United States this activity seems more concentrated around our Independence day and New Year’s Eve (although pretty rare overall). For the purposes of this posting we’ll just consider bullets fired from small arms as opposed to artillery shells. Bullets falling from the sky are no laughing matter. They can cause serious injury including death. But, there are some misconceptions surrounding the lethality and velocity of bullets returning to Earth.
One of the most common misconceptions is that small arms bullets will return to the ground at the same, or greater, velocity than when emerging from the muzzle of the weapon. Now bullets come in a variety of sizes (caliber) and shapes. Basically they are conical in shape but the nose can be flattened or rounded as can the base (to a lesser degree). Believe it or not a number of studies have been conducted on this issue since at least the early twentieth century. It was found that some common rifle calibers such as the .30-06 attained a muzzle velocity of 2700 feet per second and gained an elevation of 9,000 feet. When a bullet is fired straight up into the air it will continue on it’s flight path until it is overtaken by gravity and begins to free-fall back to Earth.
During this fall the bullet will be affected by air resistance. Now it is theoretically possible the bullet can return one of three ways; nose first, base first, or tumbling. Of the three, a nose first return from a near-vertical trajectory is the least likely. It is much more likely that the bullet will tumble or fall back in generally the same orientation (so base first) especially if it retains any of the gyroscopic spin imparted from the barrel (rifling). Tumbling creates the greatest amount of air resistance so the bullet’s terminal velocity will be lowest in this condition. Regardless, most lead core bullets will return to the Earth in about 45-55 seconds. The falling velocity will be determined by a number of factors including the air resistance and bullet weight. Generally speaking, a smaller bullet like a .22 short will have a lower free falling velocity than a larger one like a .30-06.
While the muzzle velocity for most small arms can average around 1,000 feet per second that same bullet may only reach a free-falling velocity of 150-250 feet per second. If you consider that a bullet needs to reach a velocity of approximately 200-330 feet per second to perforate human skin in a nose first orientation you can see how these falling bullets may not cause a fatal wound. The key words being “may not”. There are plenty of case studies of people being killed by downward arching bullets (though some are not at near vertical falling angles. They can still be very dangerous and cause serious injury. They will typically cause less trauma than one fired at close range however.
Reconstructing the original trajectory is nearly impossible because of all the conditions (wind, air resistance, tumbling, original load conditions, etc.) during flight that can alter the bullet path. So if you are thinking about using a falling bullet in your novel you might want to keep some of this in mind. A falling bullet might only pierce the skin or in some cases just create a bruise. In other cases it may actually cause death. The important thing to remember is that they are not traveling at the manufacturer’s listed muzzle velocity. It should go without saying that you should never attempt to conduct this type of research or experiment. All of the published studies I am aware of were conducted by the military on large scale controlled firing ranges. Do not try.
Forensic audio analysis has been around long before the Watergate tapes and continues to make advances. But a monumental leap forward occurred about a decade ago right here in Colorado. You see, one of the foundational elements of any analysis is that of authentication. Edited and doctored recordings should always be considered when reviewing audio evidence. It’s more than just verifying the identity of the source. The source being a person or activity like a gunshot, car horn, or mechanical device. It’s also about verifying the date and time of the recording. Was it made just before the commission of the crime or a year earlier? Sometimes the words said are not as important as when they were uttered. It is the when that helps detectives put things in context. It turns out that within virtually every audio recording there is a silent witness.
There is something called the mains frequency. Inaudible to our ears it is a persistent hum generated by our electrical grid. Amplify the sound and you’ll hear a metallic buzz. Researcher Dr. Catalan Rigoras discovered that this hum fluctuates as the load on the power grid goes up and down. Think of it as a melody of notes arranged in an ever changing symphony. Since the load on the electrical grid is constantly changing, so too is the frequency of the hum. So forensic audio specialists and other researchers have embarked on am ambitious project to record this hum every second of every day and cataloging the signatures in a searchable database. It’s called Electric Network Frequency (ENF) analysis and they’ve been doing it for years. So if an audio recording is made anywhere near an electrical source it will record the hum as well. It can be an electrical outlet, a lamp, computer, anything. The distinctive hum signature acts like a time stamp or watermark. It is unique to that moment in time. The longer the recording the more easily it can be authenticated.
The hum is unique to the power grid so it can get a little more tricky in countries with multiple grids in service. But if a single grid is in use the value of the analysis increases dramatically. Say for instance that someone cuts and splices pieces of audio together…the hum signature won’t match. There will be gaps in the hum when compared to the source hum in the database. What happens when two different audio tracks are combined to make a fake original? There will be two hum frequencies in the background. So unless your recording is made in the desert or the middle of the forest the recording will likely contain the distinctive hum in the background. You might want to consider using this technology in your novel. Whether your character is talking on a cell phone or being recorded on a surveillance system the hum will be there. The key is to use some kind of doctored recording. Your protagonist can then do the ENF analysis to show that the recording has been tampered with (or is original). Either way its a really cool technology.
How many times have you watched a police television drama unfold with the detective asking whether there were any signs of forced entry? It’s one of the most common initial questions in burglaries, home invasions, rape, and murder. If there was no evidence of forced entry then the speculations abound as to whether the suspect was let in, had a key, or made entry by deception. While there are ways to enter a dwelling without force (which I’ll cover later), today I’ll touch on some of the more direct methods and what evidence is left behind. There are only so many ways to enter a dwelling and most suspects use the obvious routes; doors and windows.
The most common method of defeating a door is the frontal kick. Many times this involves the suspect kicking the door near the knob or handle in an effort to defeat the bolt. It’s not as easy as it looks on television and many crooks will have to kick the door repeatedly to force it open. Another similar technique is called the “mule kick”. The suspect faces away from the door and kicks straight back near the bottom of the door to achieve the same result. They are trying to dislodge the molding around the door frame and forcing free the bolt. In either case you will find shattered pieces of the molding on the floor inside the door and the deadbolt (if engaged) will still be thrown open. With either technique you will obviously be looking for shoe impressions on the door (either midway or low) which may require the use of side lighting.
Another common method is to “shoulder” the door. As it sounds, the suspect rams the door with his/her shoulder in an effort to force the bolt through the frame. If the door or clothing is dirty or wet you may find fabric impressions. In all of these cases (kicking or shouldering) it is common for the suspect to ring the doorbell first. They want to know if anyone is home or if there are any large dogs they might encounter. So swabbing the doorbell might yield a suspect DNA profile. If the surface area of the doorbell is large enough you my even get a partial fingerprint impression.
Doors can also be pried open with the use of a tool like a pry bar. These tools may leave impressions in the door frame which can be compared to the class and individual characteristics of a tool should one be recovered from a suspect. These pry marks may also indicate the type of tool used (screwdriver, knife, pry bar, etc.) and link that crime scene to others where similar tools were used to gain entry.
Less common is the use of a ram. A ram can be anything from a sledge hammer to a vehicle. Vehicles are more commonly used in retail business theft. You have probably noticed stores with large cement pillars outside their front doors. This is to prevent someone from literally driving through the front door. This technique is most commonly seen in thefts from gun stores and jewelry stores. The suspects want to get in and out very quickly after the alarm is sounded and a vehicle (usually stolen) is ideal for smashing through heavy doors and iron bars. The stolen goods are then thrown into the car for a quick getaway before the police arrive (they hope). Of course using a vehicle in such a violent manner is akin to a traffic accident and the suspects may leave behind paint transfers, tire impressions (on the broken glass) and even broken pieces of the vehicle that can be later matched.
We often take windows for granted but unless they are bullet resistant, they don’t offer much protection from bad guys. Assuming they’re locked, a window can be defeated with a simple rock. Usually a landscaping rock from the victim’s own yard! You may be able to get a DNA profile from the rock and on a few occasions I’ve even gotten fingerprints. Suspects entering through a broken window can cut themselves (leaving blood) or catch skin or clothing on the broken edges of the glass leaving behind trace evidence.
Walls and Roofs:
Less common points of forced entry are walls and roofs. These avenues generally require more time and effort but sometimes the bad guy figures its his best way in. A wall can be breached with a ram (as mentioned above) or by cutting with power tools. Roof access is usually through dislodging a vent or small HVAC unit. Sometimes they will smash through a chimney to gain access to an attic or mechanical space. The it is just a matter of smashing through the ceiling. Of course, some suspects forget the ceiling is high above the floor and may suffer serious injuries in a fall. Like the guy in this video they may find themselves locked inside the building with no easy way out. This guy actually had to wait for the cops to arrive!
From time to time I get questions about what happens to cartridges when they are exposed to fire. Some people mistakenly believe that cartridges exposed to fire will explode and project the bullet as if it were in a firearm. This is not the case (unless the cooked off cartridge is actually chambered in the firearm at the time of the fire. Even in such events the firearm will not cycle another round (in semi-auto weapons) because there isn’t anything bracing the firearm during blow back. Hollywood has taken quite a bit of license when depicting these events so I thought you might enjoy this (rather long) video showing what actually happens when cartridges are exposed to a number of fire events. Fire fighters have to deal with all kinds of hazards when fighting fires but retail ammunition is not nearly as dangerous as other hazards even though it does sound terrifying when it goes off. After watching the video I was wondering about the first experiment with the custom mount for the cartridges. I wonder if “anchoring” one end of the cartridge actually intensified the reaction because it restricts movement of the cartridge.
It’s Winter in Colorado which means that criminals will be leaving plenty of boot impressions in the snow. CSIs face certain challenges when dealing with snow impressions. Snow is not a forgiving medium. It is difficult to photograph due to its reflective qualities and it is even more difficult to cast in. Snow comes in as many varieties as criminals I suspect. Some methods, like sulfur casting, may work well with certain snow packs while others do not. Sulfur casting presents it’s own challenges and many agencies seek a simpler solution to snow casting. One such method is casting with an aerosol wax commonly referred to as snow print wax. This wax is designed to be used with dental stone. Dental stone is an excellent casting material but it can not be poured directly into the snow. It is much too heavy and as dental stone sets it generates heat. This heat will melt the snow impression detail before the dental stone has a chance to set.
In order to use dental stone in snow you have to build a protective base layer. This base layer will capture the detail of the impression and serve to protect it during the curing process of the dental stone. Snow print wax creates such a barrier. The wax comes in a can and is propelled by an aerosol in the same way spray paint works. Once the snow impression is photographed the reddish wax is sprayed above the impression in short sweeping motions so that the wax drifts down onto the impression. Spraying the wax directly into the impression can damage the fragile features from the force of the propellent.
Snow print wax is not perfect though…far from it. The wax can not be allowed to freeze which means you can’t leave it in your crime scene vehicle for weeks on end. Most CSIs will keep it in the lab and even lay it on the dash over the heating vents on the way to the crime scene to ensure it works properly. Even in the best of circumstances the wax may clump together and sputter larger elements onto the impression instead of a finer mist. When that happens you run the risk of damaging or obscuring details in the crime scene impression. One might wonder then, why use it at all? The simple answer is ease of use. Add to that the fact that many CSIs are not accustomed to casting in snow and it’s easy to understand why they may choose snow print wax. It’s as easy to use as spray paint.
The wax is applied to the impression one shallow layer at a time until all of the impression is covered to a depth of about an eighth of an inch. You have to spray from multiple angles to ensure that all of the snow is coated (vertical and horizontal planes). The wax is allowed to dry about five minutes between coatings and I find that four or five coatings is enough in most cases. After the final coat is allowed to dry the dental stone can be mixed (with very cold water) and gently poured into the impression. In freezing temperatures it may take an hour or two for the dental stone to fully set. Once it has, the cast can be lifted from the snow pack and booked into evidence.
Footwear impressions can provide valuable information to the crime scene reconstructionist. Examiners look for class and individual characteristics in crime scene shoe impressions that may link that impression to a particular suspect shoe. In order tom compare a crime scene shoe print to a shoe though, it’s best to make an impression from the suspect shoe. There are a number of ways to take footwear impressions. One can use black ink or fingerprint powder but, a novel method is the use of an inkless pad. The process involves a pad coated with a special “ink” and chemically treated sheets of paper large enough to hold a boot impression. They work great for footprints as well (no one has yet developed one for taking tire impressions). The ink pad is generally yellow in color, odorless, and can be used for several hundred impressions before it needs replacement. You have to use the chemically treated paper however. The process will not work with standard bond paper because of the chemical treatment. The paper isn’t cheap either. A pack of one hundred sheets costs about forty dollars (US). Initially, the inked impression may appear green in color but will dry black. This provides the highest contrast with the white paper.
This process is well suited for taking elimination impressions. Elimination prints are those from people who may have been in the crime scene but are not suspects. This includes victims, family, customers, employees, etc. The inkless method means you don’t make a mess of everyone’s shoes or run the risk of them tracking traditional ink or fingerprint powder all over the crime scene. It also means you can take very good impressions without having to collect the shoes. Now occasionally, a victim will be wearing shoes very similar to the suspect. In such cases the CSI will have to collect the shoes and book them in as evidence unless they are easily distinguished in size. For everyone else though, this inkless method works very well. All of this is in addition to photography of course. You might be asking yourself…why take impressions if you’re taking photographs? The simple reason is that an impression is a full scale representation of the outsole. Photographs may be slightly out of focus or at an odd angle so that when you enlarge them to life size they do not align correctly. The link below is to a short video demonstrating this process. The only thing I would add is that it is better to wear the shoe and step onto the pad. The weight of the body imparts a better impression in my opinion.
I generally make it a point not to discuss certain aspects of bombing investigations such as the design of improvised explosive devices, formulas, detection capabilities, tactics, or render safe procedures. I avoid these topics to keep my colleagues safe and the terrorists in the dark. However, I can tell you a little about how we collect evidence from such events. We refer to them as post blast investigations and they can be very complex. As such, I’ll limit the discussion to some of the more obvious and common techniques. Not all CSIs are trained in post blast investigations so keep that in mind when you are developing your character. Agencies that deal with terrorism regularly, like the FBI, IDF (Revidim), Metropolitan Police, RCMP and many others, are more likely to have post blast investigators as opposed to small local municipal police agencies. However, in the United States, many sheriff and larger police agencies have bomb squads and post blast investigators thanks to the efforts of the FBI.
Now there are a number of ways to build a bomb and just as many different types of explosives but they all have one thing in common. They all go boom (unless the good guys take care of it first that is). The spot at which the device detonates is called the seat of the explosion. Contrary to what you may have read an explosion doesn’t typically destroy all of the evidence. It may damage and disperse the components over long distances but good investigators can always find something. So to keep this embarrassingly simple let’s think of evidence recovery from three different sources.
The first is the actual seat of the explosion. With large ground based devices this is typically represented by a crater. The larger the device the larger the crater (usually). Large vehicle bombs can leave a crater the size of a house while something like a briefcase may leave a much smaller crater. It’s not uncommon to find pieces of the device still in the crater. Of course, all of this is dependent on the type of explosive used. Regardless, traces of the explosive used will be found in the crater. Post blast investigators can take samples of the soil from the crater and seal it in air tight containers (to keep certain elements from escaping in gaseous form).
The second type of collection site is from surrounding surfaces like trees, buildings, vehicles, clothing, traffic signs, even victims at autopsy. You see, explosions can release a lot of energy. That energy creates a shock wave (much like that produced out of the muzzle of a gun when fired) which can carry explosive elements and depositing them onto surfaces as a residue. Think of it like soot from a fire. This video link will demonstrate the appearance of a shock wave following an explosion. Explosive Shock Wave Video. Investigators can sample this residue with everything from small cotton tipped swabs to larger cotton pads. The samples are also sealed in air tight containers to ensure the volatile compounds do not escape.
The last type of location is from components of the device. If you’ve ever read about notorious bombing scenes you know that parts of the device can be found many blocks from the seat of the explosion. It may be anything from pieces of a vehicle to small electrical circuit boards from the timer. These components were obviously in close proximity to the device and should have explosive residue on them (assuming they are collected before environmental degradation). If the items are small they can be placed in air tight containers. If they are larger (like a vehicle axle) they can be swabbed.
Obviously, I’ve left a lot out. I haven’t even touched on sensitivity of detection for obvious reasons. Bombing scenes can be very challenging and dangerous. It’s even worse in a war zone. But even in a war zone forensic evidence can be collected and analyzed. As with all crime scenes you just have to know where to look for evidence. In bombing scenes it can be almost anywhere. Rooftops, the sides of buildings, tree tops, even rain gutters ad sewers (following fire suppression efforts). There’s a ton more that goes into these investigations but that information is best kept secret.
I’ve written before about the three types of crime scene photographs. Well, those photos are the ones taken at ground level. Photography is all about perspective and sometimes you need to get off the ground. Most of you are probably aware of aerial photographs taken from rotary or fixed wing aircraft. I’ll post something on historic and modern aerial photography soon but there is a simpler and more widely used practice to obtain a heightened perspective. We call it elevated photography. There are a number of ways to obtain an elevated perspective. All of them are dependent on the conditions on scene and the available equipment. Of course, these techniques can also be used in the crime lab to photograph large items like bedsheets or large areas of carpet. Here are a few examples.
This is a simple product marketed for crime scene investigators. Basically it’s a pole with an attachment for the camera. Fancier models also include a shutter release cable to trip the camera and take a picture. The best results require the operator to manually focus the camera lens. Since the operator can’t see through the view finder you usually have to make several test shots to achieve the one you want. These booms are typically employed in places like bathrooms or bedrooms where there is limited space or height to elevate the photographer. They can also be used to photograph objects in elevated positions like bird nests or tree limbs.
The simplest is a six foot bi-fold ladder. A CSI may also use scaffolding or a taller ladder. In the extreme you might use a fire truck ladder extended out over a body or vehicle in a field. Of course, the higher you go the more risk you’re exposed to. I remember the first time I went up on a fire truck ladder. I was taking pictures of a woman’s body that was dumped in a field. There was no harness and pretty strong winds. I was pretty scared. It seemed to take hours and I couldn’t bring myself to get more than forty or fifty feet in the air. These days, safety is taken more seriously and after a few trips up you get used to it. Of course, if you hate heights you may have to call in a favor from a co-worker to take your place.
An Elevated Platform:
Any elevated position can work as well as a ladder. It may be the landing above a living room or the rooftop of an adjacent building. Some larger crime lab vans also have special roof platforms designed to accommodate a photographer. Fixed locations obviously limit the options for the photographer but they may be better than nothing.
In case you missed our live chat last Friday night with the guys and gals of the Crime Writer’s Panel I thought it might be interesting to expand a little on the topic I chose to cover. Don’t worry, the panel’s discussion will be posted to YouTube soon and I’ll tweet the link or update this page. The panel was comprised of a bunch of really talented and experienced crime writers and former law enforcement and it would be worth it to watch. There’s a good chance we’ll get together for another chat session and I’ll let you all know when that is. We spent the hour talking to fans about the inside scoop on a variety of topics in law enforcement and forensics. The topic I chose to cover comes from one of the most frequent questions I get from crime writers; what is the perfect crime? The question always gives me a moment of pause because I take care not to reveal sensitive information publicly. It’s a great question but, the answer may surprise you a little.
Some authors spend a lot of time trying to design the “perfect crime”. They develop intricate plots carried out by sophisticated criminals weaving plot twists and turns throughout each chapter. Real life works differently and understanding why some crimes go unsolved may help you develop a more realistic story. I’ve investigated thousands of crimes and I have yet to encounter a “sophisticated” criminal. It’s really hard to plan for every contingency, anticipate every step taken by the police. Most detectives and CSIs spend much more time studying the aspects of crime than the criminals committing them. It’s our job. To me, a perfect crime is one for which there is no evidence of it ever having happened. No evidence, no missing person, no motive, nothing. That makes for a really short book! So instead of trying to come up with your version of a “perfect crime” it may be more worthwhile to understand why some crimes go unsolved. There are basically three reasons.
Lack of Probative Evidence:
Most judicial systems require evidence to prosecute a criminal. In the United States we have a very high legal standard to meet in order to obtain a conviction. All of the burdens are placed on the prosecution. That said, it’s kind of nice to have a little evidence to work with. The less evidence we have the harder it is to prove guilt beyond a reasonable doubt. There are two conditions that contribute to a lack of evidence. The first is limited activity. Activity creates actions and actions create evidence. Take a drive by shooting with a revolver for example. If there are no witnesses (cooperative or otherwise) then all you may have is the fired bullet. Depending on the terminal ballistics the bullet may be too mangled to retain usable rifling characteristics. Even if you were to retrieve an identifiable bullet you have to still find the gun, and from there, the shooter. This leads us to the second reason; the passage of time. Most evidence degrades to some degree with the passage of time. You may have a female victim raped and strangled in the woods but if decomposition reduces those remains to skeletal elements then you won’t likely recover DNA or other trace evidence to help identify the killer. Evidence can also be missed but, this rarely happens with seasoned investigators.
Over-abundance of Evidence:
This may sound counter-intuitive but there are times when you can have too much evidence. Consider the murder of a prostitute who was also selling drugs and stealing from her pimp. Then you find her dead. Was it one of her Johns? Her pimp? A rival drug runner or prostitute? This scenario creates multiple suspects, motives, and possible opportunities depending on the known timeline. Even if the police develop what they consider a “solid” suspect, the existence of other “possibilities” may complicate the prosecution. In other cases you may find trace DNA that is not associated with any plausible associates. Even if you have suspect DNA in a suspicious location; the presence of “stranger” DNA may stall the prosecution. It’s not a given, just a possibility.
Misinterpretation of Evidence:
Despite what you see on television this is the least likely of reasons why crimes go unsolved. It happens. We are all human and humans are subject to making mistakes or misunderstanding evidence of an event. It may be a lack of training or maybe the detective is operating on very little sleep. A suicide looks like an accident or a murder is staged to look like a suicide. Staged crimes are very popular in fiction. For one reason or another I’ve had the opportunity to investigate a number of staged crime scenes. Everything from burglaries to kidnappings, to suicides, and more. You may find this interesting but staged crimes are usually not difficult to detect. The reason is simple. Most stagers don’t understand crime the way we do but, it is always a concern.
So there you have it. Three reasons why crimes may go unsolved and therefore could be considered “perfect” crimes. These categories are very broad and may overlap. Of course, I didn’t even touch on issues like backlog, training, or time constraints but I’ll save those for another day. The bottom line is that the universe dances to its own tune. We can practice and study for that big recital but in the end we can’t control every outcome. Such is the nature of life and so too is the nature of death. Despite our very best efforts the deck may have been stacked against us from the beginning.