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Touring The Crime Lab: The Fingerprint Section

The fingerprint section is a workhorse of many crime laboratories and much more prevalent than DNA sections. Even small agencies, like university police departments, can have a fingerprint section. Many modern crime laboratories are divided into two broad areas; laboratory and administrative. Administrative areas include the offices or cubicles of the analysts as well as conference rooms, break rooms, etc. while laboratory areas are designed for work with hazardous materials and evidence. Most evidence is examined in the laboratory areas but latent fingerprint cards and fingerprint arrest cards (also known as 10 print cards) might be compared at the analysts desk. As such, I will talk about the fingerprint section in terms of the laboratory area and administrative area.

The Laboratory Area:

This part of the fingerprint section is where all of the physical processing takes place. Fingerprint examiners use a variety of chemical reagents and powders to develop latent fingerprints and need an area to conduct these process safely. Because powders and liquid reagents don’t mix well and the nature of air flow there are generally two types of work stations. The downdraft hood is used for powder processing.  This type of hood draws air (hence powder) down and away from the analyst. A filter system traps the powder so it doesn’t get airborne in the general laboratory area. Fingerprint powder isn’t considered really hazardous so using a hood isn’t required. It just makes things less messy.

Down Draft Fingerprint Hood

Chemical fume hood with glass shield

Chemical reagents are used under a fume hood. These hoods have a much greater air flow (at least 100 linear feet per second across the face) drawing the air up and away from the analyst. These fumes are filtered before exiting the building.  Many fingerprint sections also contain a photo station with a copy stand and alternate light source so that fingerprints can be professionally photographed as they are developed. Countertops and sinks are generally chemical resistant and dark colored in modern laboratories. Obviously you’ll also find lots of drawers and cabinets containing supplies and chemicals used in this section.

Another common piece of equipment is the superglue fuming chamber. You can see a “bank” of these cabinets behind the man in the video. These fuming chambers expose evidence to cyanoacrylate fumes thereby making the latent fingerprint visible. The fumes are very irritating and these cabinets need to be able to evacuate all of the fumes before the doors are opened.

Superglue fuming chamber

Heat plate and bottle of liquid cyanoacrylate (superglue)

The Administrative Area:

One would think that most of the work in a fingerprint section is done in the processing lab. However, examiners spend most of their time doing comparisons and computer searches through AFIS like systems. These areas may also contain printed arrest cards from suspects arrested by that agency (as well as employee fingerprint records).  This work might be done at the examiner’s desk or a shared workspace. Either way it is a typical office setting. AFIS terminals are best housed in a room separated from other sections so that the lights can be turned off. This allows the examiner to see the screen images with better clarity.

Examiner using an A.F.I.S. computer



Conflict Between IT and the Crime Lab

Okay, maybe it’s not a war but we sure do seem to piss each other off quite a bit. I don’t envy the job the information technology folks have to do in law enforcement. It’s a pretty much thankless position where everyone demands 110%; 120% of the time! Most municipal governments made the decision years ago to employ their own people to manage their computer networks instead of farming it out. The logic is pretty simple. Governments have secrets and law enforcement has some of the most sensitive information within the local government. The last thing we want is to have any of that sensitive information (like photos of a young girl during a sexual assault examination) leaked unless required by law.

Information is compartmentalized even further within a law enforcement agency. The CSIs aren’t going to be able to access the confidential information in the drug task force and those folks generally won’t have any access t0 crime scene photos unless it’s one of their cases. Information is shared on a need to know basis. The one constant is the IT specialist. They have to get into everything sooner or later. The point is that different sections of the police department have different computer requirements. Our computer needs in the crime lab are different than those of the gang unit because we use these tools in different ways.

Most forensic scientists pride themselves on being intelligent. We all own home computers and we’re pretty savvy about the software we use. However, many labs aren’t allowed to manage their own systems. You can’t load your own software or even purchase it without the blessings of IT or Presidential mandate. Most of the time you’re not even allowed to de-frag your hard drive (you could unknowingly set the thing on fire, or something). We’re intelligent enough to reconstruct complex crime scenes but not smart enough to perform the simplest of functions we do at home. Ironically, the same is true of the computer forensics division whose members employ the it’s better to ask for forgiveness than for permission philosophy.

Now before you IT groupies shower me with hate mail let me say I understand the need for compatibility. Downloading foreign software  onto a network can have catastrophic consequences. The problem is that IT tends to promote a one size fits all model for the entire department. In terms of “computer profiling” the crime lab is lumped together with the detectives, records management folks, and the secretaries. This poses a number of problems because we have special needs in the laboratory.  It’s common to have our internet traffic blocked or fight for months to get approval to download specialized programs. In the end I think it would just be simpler to have our own stand-alone network.

Here is one funny/tragic story that may help exemplify our needs and the dangers of using the one size fits all model. I once worked with a video expert who used an AVID computer system. This is a high end computer system for processing and enhancing audio and video evidence. She is freaky good at her work and a nationally recognized expert.  A day came when she had to upgrade the Windows operating system on the computer. The company sent her detailed instructions on how to do the upgrade to make the data transition run smoothly. She provided the instructions to the IT staff member assigned to the project, gave him verbal instructions, and went to lunch. When she came back she had a new operating system and a reformatted hard drive.

The problem was that this system did not have external back-up drives available so three years of case work was erased. She was a wee bit pissed as you might imagine and things got worse when she found out the IT guy didn’t follow the instruction provided by the manufacturer. His response was something along the lines of I know how to upgrade an operating system which a century ago would have earned him a showdown on main street at ten paces! He looked at our sophisticated video system as just another computer and we paid the price for it. The original evidence was still available but all the work done to enhance the videos for those cases had to be re-done on a case by case basis. It sucked.

You would think that most CSIs get burned out by the doom and gloom they see every day but a lot of us are more bothered by stuff like this. All the little rules and regulations and policies that make a difficult job pull-your-hair-out tough!  If you are writing a story with a CSI you might consider issues like these as additional stressors that may influence your character relationships.

Rare Genetic Mutation Prevents Fingerprint Development

Below is a very interesting video interview of a researcher in Israel who has documented a genetic mutation that prevents fingerprint patterns from being formed. The mutation is extremely rare (known to occur in only four families worldwide) and not likely to impact criminal investigations but it is interesting nonetheless. The mutation is referred to as adermatoglyphia and affects the SMARCAD1 gene which apparently regulates the formation of fingerprint patterns used for comparison. Criminals have made attempts to alter their fingerprints to avoid detection but this mutation wouldn’t leave the physical scarring that we normally associate with such events. If there were a lack of bifurcations and ending ridges then the prints couldn’t even be entered into AFIS.

Because the mutation is genetic there is the possibility that multiple family members may share the condition. That could prove very interesting for a storyline. Imagine how this condition might help members of a crime family? Imagine further if identical twins shared this mutation? They would have indistinguishable fingerprints and identical DNA!! Talk about a who-dunnit! Even if the police found clues to implicate the family it would be tough to sort out who did what by traditional forensic examinations.

If you write spy thrillers you might consider using this condition for your character as well. Imagine if an intelligence agency actually recruited individuals with this condition. For a sci-fi novel maybe someone clones individuals with this mutation for nefarious operations. You can really have some fun with it. They represent that once in a lifetime case most examiners dream of getting. I think it would make for a fascinating twist in your story.

What The Heck Is Magnetic Fingerprint Powder?

Most people are familiar with the concept of fingerprint powders even if they have never used it or had it spread all over their home. It’s cheap, usually black, messy, and pretty hard to clean up. It works well though and most basic processing kits used by CSIs and patrol officers contain a jar. Aside from black it comes in a multitude of colors (made to contrast with various backgrounds) as well as fluorescent powders for use with an alternate light source. But there is another type of fingerprint powder you may not be aware of and we call it magna-powder. Magna-powders come in all the same colors as traditional powders but what makes them different is the presence of tiny magnetic flakes embedded with the powder.

Magnetic Fingerprint Wand. The magnet is on the left end and the plunger is on the right

Magna-powder is more expensive initially but it can pay for itself over it’s lifespan. What makes magna-powder advantageous at times is that the “brush” never contacts the evidence (as it does with a fiberglass brush) which may help with fragile prints. The photo above shows one type of “wand” that is used to apply the powder to the evidence. The tip of the wand houses a magnet that “attracts” the powder to the wand holding it in place while the analyst brushes the evidence. Another great aspect of the powder is that any unused powder can be collected with the wand and placed back in the jar for processing at a future date. The examiner just pulls up on the plunger at the other end of the wand which separates the magnet from the outer housing and the powder drops back into the jar. This makes for a more effective clean-up which any home owner would appreciate.

Developing fingerprints with magna-powder

Magna-powder can be used on a variety of surfaces but I have had the best luck on plastics (like credit cards and driver’s licenses). It can also be used on paper and I have had success using it on water soaked documents that have been dried (as in recovered from lakes). Obviously using it on ferrous metals or magnetized surfaces might pose some challenges.  There’s nothing particularly special about the powders but having your characters break out a jar might give your story that extra detail that others will miss. Certainly, any readers who are in law enforcement will recognize that you’ve done some homework by picking a powder few other authors talk about.

AFIS: The Automated Fingerprint Identification System

Fingerprint Examiner Searching with AFIS

Many of you may be familiar with the term A.F.I.S. (automated fingerprint identification system) but may not fully understand how it works. Contrary to the name it is not, in fact, an identification system. It is a searching system. Computers do not make an identification like you see on television, that is done by the fingerprint examiner. Computers can narrow the number of people (called candidates) but examiners have to make the final call. Part of this has to do with the way the systems are designed.

My experience comes from being an examiner here in the united States but these systems are in use in other countries like Australia, Europe, and Canada (although they may organize their records differently than we do). Fingerprint databases may be composed of criminals and non-criminal candidates which will be determined by the laws of that jurisdiction. In the united States we have three tiers of searching which may seem redundant but there is a method to the madness. First, some local agencies may have their own local database (offenders arrested in that city). Having a small database means you can search a fingerprint very quickly. The downside is if your offender hasn’t been arrested i your city.

We also have a state level database which holds the fingerprint records of all offenders arrested in the state. It is a larger database and so it takes longer to conduct a search.  A rush request (for a major crime like homicide) can complete a search of several million records in under 20 minutes if the print quality is good.  A low priority search will usually be completed within a few hours.  In the United States we also have a federal database (called I.A.F.I.S) which holds all arrests in the country. Again, larger database equals longer search times.  As with all databases the old axiom “garbage in / garbage out” holds true.

Screen Shot from an AFIS Examination

So how does a search work? The examiner begins by scanning an image of the fingerprint. Once uploaded, the software allows the examiner to mark the locations of ridge characteristics (bifurcation, ending ridges) with a “marker”. The examiner can also add data to the search such as the suspect gender and the finger (ex. right index) if known. The examiner also defines the pattern type (if known) and marks the location of the core area. These markers make a kind of constellation which the computer uses to look for similar patterns of the people already in the database. You see, when a record is entered the examiner marks the very same characteristics for the master record.

The downside is that when prints are poor in quality or incomplete in pattern area then the examiner may be “guessing” as to things like pattern type. So they may think the suspect print is a loop but it is really a whorl. In practical terms, when dealing with a partial fingerprint the examiner may just search it as multiple patter types or all pattern types but this will bring back a large pools of candidates.

Unlike television, when the candidates come back the computer does not flash the word “match” on the screen. The software will “score” the candidates from best to worst based on how well the markers correspond (which is determined by a complex algorithm).  The examiner has to compare each one individually. Even if the examiner declares a “match” in the system they typically have to get a copy of the record from the state for a final confirmation and verification of the identification before writing a report. This protects the examiner from any images which were incorrectly entered from the “known” suspect card.Many systems will also search palm impressions but others do not.

Remember, if the data is entered improperly then the fingerprint may not make the candidate list for comparison. An analyst may enter the wrong pattern type, gender, or perhaps the quality of the original impression is just too poor to be of value. These prints should get weeded out but some make it through.

How Are Fingerprints Compared?

Fingerprint Comparison Chart

Fingerprints have been used to identify criminals since at least the late 19th Century. The process of comparing fingerprints is fairly straight forward however, there are a number of conditions that examiners have to take into consideration before reaching a conclusion. I’ll cover some of those issues in future posts. For now, let’s focus on the basic methodology and terminology. The fingers, palms, toes, and soles of our feet are covered in unique arrangements ridges called friction ridge skin. These ridges are the black colored lines you see in inked fingerprint impressions. The ridges are separated from each other by furrows (the white lines). This arrangement of friction ridge skin is fully developed in the third trimester and will remain unchanged (except for size) until death barring serious injury like deep scaring or burns.

Latent fingerprints from the crime scene can be compared to other latents but ultimately an examiner compares them to the known inked fingerprints of a person. This is the only way to say for sure if the fingerprints came from a particular The first step is to examine the fingerprint pattern. If the latent fingerprint pattern is different than that of the suspect’s then they can be eliminated as the source of the fingerprint. Even if they share the same general pattern type one may have a distinctly different arrangement of details.

These details are referred to as 2nd level detail. Historically they have been known as minutiae, points, and Galton’s points (in reference to Francis Galton an early fingerprint scientist). These details are generally named bifurcations, ending ridges, enclosures, short ridges, and dots.  Bifurcations occur when a single ridge splits into two distinct ridges. An ending ridge is what it sounds like; a ridge ending abruptly. An enclosure occurs when a ridge bifurcates into two ridges, travels a short distance, and then re-converges back into a single ridge. A short ridge is what it sounds like and a dot is a single ridge element “floating” by itself.  Obviously, the fingerprints being compared have to have a sufficient number of these points visible to be of value for comparison but for the sake of this posting let’s assume that they do.

A bifurcation where one ridge splits into two

An ending ridge

An enclosure of friction ridge skin with "doughnut" appearance

A short ridge

A dot of friction ridge skin

With the latent and inked fingerprints side by side the examiner compares the relative locations and arrangements of these points to each other.  If possible the examiner will begin their analysis at the core (indicated by green arrow in figure) or delta (red arrow). From these locations they can count the number of ridges in any direction to the details. If an examiner finds that there is a bifurcation opening to the right six ridges above the core then they would look for the same detail in the same location on the other impression. This process continues, point by point, until the examiner finds an unexplainable difference (meaning the two prints were made by different people) of they find sufficient agreement between the two impressions to declare an identification.

The Core (green arrow) and Delta (red arrow) of a loop fingerprint

More on the conditions affecting this process in later posts…

How Does Superglue Fuming Work?

Modern fuming chamber with humidifier on right side

Superglue fuming is a chemical process to reveal and “fix” latent fingerprints on non-porous items such as bottles, firearms, knives, etc.  The process work by heating liquid cyanoacrylate (i.e. superglue) which releases gaseous vapors that adhere to the oily residue of the fingerprint. In the laboratory this is done in a fuming chamber. When I first began my career this amounted to a fish tank.  It was simple, crude, and messy. While I suppose some smaller agencies may still use these tanks the vast majority of modern labs use a chamber designed specifically for this function. These chambers control humidity and heat activation. Humidity is an important factor to control because the moisture in the chamber helps to re-hydrate the fingerprint residue. They also evacuate the dangerous fumes so the examiner isn’t exposed to them when they open the chamber up. With the old tanks we’d throw open the lid under a fume hood and run so as not to be exposed to the dangerous vapors.

Superglue developed fingerprint

Examiners can also use a fuming wand to expose items to the cyanoacrylate vapors. They are basically butane fueled lighters that heat small brass capsules of cyanoacrylate infused packing. These wands are typically used in the field on larger items that can not be moved or placed into the fuming chamber. Care must be taken not to inhale the vapors and a safety mask is recommended for use. Also, since the wand can get very hot you don’t want to lay it on anything flammable.

CSI using a fuming wand in the field

Upon development the fingerprint ridges will appear a white chalky color. In this state they may be photographed, treated with fingerprint powders and lifted, or treated with liquid dye staining reagents and photographed with an alternate light source. The impressions are much more durable than in their untreated state which means an examiner may have an opportunity to make a second or third tape lift if need be. All in all, superglue fuming is one of the most common techniques in the laboratory for processing non-porous items for fingerprints.

Who’s Fingerprints are Really on File?

It may surprise you to find out that not everyone’s fingerprints are in a searchable police database. State and federal statutes define (by profession or crime) whose prints can be collected and accessible during criminal investigations. Aside from criminals these include teachers, child care providers, securities professionals, law enforcement, concealed carry permit holders, and some others. What may surprise you is whose prints are not directly searchable by law enforcement.

In many states like Colorado, the police can not search the fingerprints in the department of motor vehicles, employers who may collect fingerprints for internal security measures, even witnesses or victims of crime that are collected for elimination purposes during an investigation. Obviously, police can make requests for specific records from these sources but they must know whose prints they want to access. Police can’t just “plug-into” their database and run a search.

State agencies collect these fingerprints and share them with the Federal Bureau of investigation who is the main repository for these records. The FBI has developed a program to access their database by local law enforcement so even a small agency can search the entire federal database for criminal records.


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