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Forensic fun with fingerprints–Part 1

From Scott:

The television show CSI has moved fingerprint investigation into the forefront of many stories of crime fiction. It seems like on every episode of the popular TV series, the investigators magically uncover a fingerprint, and thirty minutes later the crime is solved. Unfortunately, reality follows a starkly different path. Fingerprints are hard to come by, quality fingerprints even more so, and usually at that point the print ends up belonging to the victim. That being said, this blog entry will delve into the world of latent print investigation.

The first thing I should do is break down that phrase: “latent prints.” I used the word “prints” rather than “fingerprints” for a reason. Palm prints are another means of identifying suspects, and they make up roughly one in three prints recovered at a crime scene. I’ll discuss them in depth in a few moments. When I say “prints,” I’m referring to both fingerprints and palm prints.

A latent print is one that is invisible to the unaided human eye. For that reason, we need to use one of the myriad latent print investigation techniques available to us to develop the print and make it visible. On the other hand, a patent print is one that is already visible. Usually, this means that the person who left the print had something on their hands. You’ve probably seen this before when a child grabs your refrigerator door with chocolate- or ketchup-covered fingers. At a crime scene, this includes the “smoking gun” of fingerprint evidence: one left in the victim’s blood. To borrow a phrase from Desi Arnaz on the TV show I Love Lucy, if the suspect has the victim’s blood on his hands, he has some ‘splaining to do. The other type of print is the plastic print. This is left behind when someone grabs a pliable surface and leaves an easily visible print behind. If you want to see this one for yourself, just grip a ball of Play-doh and look at the results.

The original means of identification used by the government was the Bertillon system, which started around 1883. It was developed under the theory that no two people have the exact same bone measurements and that criminals had certain bone features that indicated their propensity for illegal activity, such as small eyes that are close together. Officials at Ellis Island even used the system to deny immigrants entry into the country, believing they would end up breaking the law.

This system ran into a roadblock in 1901 in the city of New York. A man who was arrested for armed robbery denied being a repeat offender. However, a check of state records indicated that he had been arrested before. They checked his bone measurements and found they were the same as what they had on file for him. The problem: the man listed in their files was still in prison. Same name, same bone measurements, different person. They tried the Henry System of fingerprint identification, developed by Sir Richard Henry in 1897. The two men had completely different fingerprints. In the years since, fingerprint identification has become the standard in every jurisdiction. To date, no two fingerprints have ever been found to be the same.

The typical latent print is left behind by the water, sweat, oils, and other secretions on the skin. They form the pattern of the friction ridges on the skin. These friction ridges are on the palms, fingers, and bottoms of the feet. They serve an important function: providing a means of gripping things. Without friction ridges, it would be difficult to pick up anything with smooth sides. Keep in mind that just because we found a person’s fingerprint does not mean that person committed a crime. All it indicates is that the person has touched something.

There are three basic fundamental principles for latent prints. The first is that fingerprints are an individual characteristic. This is a scientifically accepted principle and has been accepted in court. Unless this is ever proven to be wrong, it will always be accepted. This makes fingerprints more exact than DNA. Remember, identical twins have identical DNA, making it impossible to distinguish between them. But those same twins have different fingerprints.

The second principle is that fingerprints never change. They are formed sixteen weeks after conception, and they remain that way for the individual’s lifetime. They can be overwritten by scar tissue, but the print not covered by the scar remains the same. Even if the skin is damaged to the point where it comes off, when the area heals, the print will return to the form it had before the injury.

The third principle is that fingerprints have characteristics that can be classified for easier identification. Without boring you by going in-depth, there are basically three types. About 60-65% of fingerprints are loops. The friction ridges come in from the side (usually the pinky side), loop around, and leave the way they came in. A whorl (30-35% of all prints) basically resembles a spiral pattern. And a tent pattern (5% of all prints) features friction ridges that simply enter on one side, rise up in the middle, and dip down to exit on the far side.

When trying to match a latent print to a set of known prints on fingerprint cards, those classifications can speed up the process. If I have ten suspects, that’s one hundred fingerprints to look at. But if my latent print is a whorl, I can eliminate almost 3/4 of the possible prints right away. Or if it’s a loop, I can still eliminate most of the known prints just by looking at which way the loop slants. For cases where there are no suspects, there is a different process. In years past, fingerprint examiners would classify the latent print based upon several features, including which of the three classes it fit into, which sub-class of that print type, and the number of ridges between certain features within the print. Then they had to go through thousands of print cards and check each one to see if there was a match.

Now, a computer system makes the process easier. The Automated Fingerprint Identification System (AFIS) stores millions of fingerprints in a nationwide database. When an unknown print is recovered from a crime scene, if it’s of sufficient quality it can be entered into the AFIS system. This computer will then determine several possible candidates, and the examiner’s job is then much easier. Sometimes, when a suspect is arrested for the first time, several older cases with unknown fingerprints on file can be solved.

Once a potential matched set of prints has been established, the examiner has to look at the tiny details called minutiae. Again, without delving into too much mind-numbing detail, we are looking for any spot on the print where a friction ridge begins or ends, where one splits (a bifurcation), or where there‚Äôs a very short ridge called an “island.” In the case of a highly detailed latent print, we can also use sweat pores as a means of identifying the print. We find several of these minutiae on the latent print first, then see if they match up with the known print. We never start with the known print and work toward the unknown, because you might force yourself to imagine seeing something that isn’t there.

Many people ask how many of these points of minutiae need to match for a print to be considered an identification. The short answer is: as many as you can. Some departments use an actual number. It could be as few as eight, or as many as fifteen or more. The current standard being taught is that the number varies depending upon the quality of the latent print. The better the latent, the more minutiae you should be able to match. In the end, it comes down to the experience and training of the person making the identification. After an ID is made, it has to be confirmed by another person before it goes any further. If an examiner ever announces an identification that is later shown to be in error, that examiner’s career is over.

As for the technique for studying the prints, we typically photograph both the unknown and the latent print. We use Photoshop to remove the color, which makes the minutia more visible. Then we go into the latent and put dots at every point of minutia we find before going back to the known to see if those same spots are there. When finished, the picture files are saved and placed into evidence, in case courtroom testimony should be needed.

As I said at the beginning of this blog, we don’t often make a fingerprint identification. The proliferation of TV and movies have taught criminals that fingerprints are the way they get caught, so they tend to wear gloves. However, we do make the occasional match, usually on an unplanned crime (such as a crime of passion, like most murders). Recently, we had a teacher at a high school accused of taping his cell phone up in the bathroom and using it to take pictures of teachers. I developed fingerprints in the duct tape that was on the phone, and we matched it to the suspected teacher. Finally, success! And then our forensic computer expert downloaded the pictures from the phone. Sure enough, there was a picture of the suspect taping his phone in place. And there goes my thunder…

Palm prints present another challenge. We sometimes get a section of palm print that’s the size of a quarter. How do you look through ten suspects’ palm print cards and find that small area of latent ridge detail? The answer: more training. Each area of the palm has ridge flow patterns that indicate what part of the palm you’re looking at. The thumb side of the palm is the THENAR area, the pinky side is the HYPERTHENAR, and the area up near the fingers is called the INTERDIGITAL. Once we learn to recognize the telltale signs, we know which part of the palm to look at. This speeds up the process considerably.

Next time, I’ll go into detail on how fingerprints are collected from various surfaces (if you want to use cop lingo in your book, the surface is called the substrate). I’ll also discuss other types of evidence, such as blood spatter.

[RICK SAYS: Vampire wannabes and fans of the TV show Dexter will want to tune in for that.]


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