Crime Labs

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This section covers what sort of things can be done in crime labs. There is not a lot of detail, since few stories need to show exactly what is done in the lab. What we usually need to know is what evidence the lab people are likely to be able to find for Mulder and Scully, and how long it will take to get the results.

Important note: This whole section has been compiled from books and websites, with occasional help from people working in this field. Like the rest of Deep Background (of which this is only a small part), this information is designed for writers of X-Files fiction, and covers only those things that they might be expected to need. It is not a comprehensive guide to the subject.

It is essential that all evidence is fully documented at all stages of its life. This is called the Chain of Custody. Whenever anyone handles the evidence, whether at the scene, in the lab etc, they must document it. If there is any time unaccounted for the evidence is inadmissable in court.

In "Grotesque," Scully looks at the bag that held a knife. We can clearly see the form on the side for people to fill in if they handle the evidence. When Mulder (and Patterson) took the knife it was a major offense as it would make the knife inadmissable evidence.


For an X-File treatment of fingerprints, we see Mulder collecting them in "Squeeze" and see the FBI fingerprint expert in "Grotesque."

Unique: Fingerprints are absolutely unique, so they remain the least ambiguous way to identify somebody, whether a dead body or a suspect. Even after scarring and injury they always grow back the same.

How prints are made: Usually in one of three ways:

  • Pressed into a soft substance like soap
  • Made by dirty fingers eg blood-stained
  • Made by the greasy secretions of normal skin.

The first two are visible and can be photographed. The last one usually isn't visible to the naked eye. They are called latent prints and must be enhanced first, using powders and other chemicals.

Obscuring fingerprints: Actually, gloves don't always prevent you from leaving a fingerprint.

Surgical gloves, for example, are designed to fit very very tightly so surgeons can pick things up during operations. Fingerprints can this pass through them. They can also be turned inside out to yield fingerprints from the inside surfaces.

Leather gloves can be treated in the same manner. Also, leather gloves can leave a print that is unique to that glove and no other (leather comes from cow skin, which is just as random as human skin).

Cloth gloves and mittens can also leave a distinctive print that can be traced back to the mitten that made it.

Collecting fingerprints: Different surfaces require different techniques for developing prints. Prints can be taken from virtually all surfaces.

  • Powders: Powder is dusted over the prints. Particles stick to the print and can then be lifted off with tape. Carbon powder can be used on light surfaces; aluminium powder or lanconide can be used on dark surfaces. Powders are quick and cheap, but messy and get in the way of other evidence. They can only be used on firm smooth surfaces.
  • Iodine vapour: This makes the print develop a brown image. Can't be used on metal, and fades fairly quickly. Also only picks up recent prints.

There are special processes that develop prints on paper, wood and cardboard. Fingerprints can be developed on objects that have been in water.

Prints can be developed off skin (such as from the neck of a strangulation victim), although not very well. Blood-stained prints are one thing, but latent prints tend to vanish when the skin shrinks after death.

Uses of fingerprint analysis:

  • Confirming identity of a suspect in custody, if he already has a criminal record.
  • Comparing prints of a suspect in custody with prints left at crime scene.
  • Matching prints found at the scene with a known criminal on record (though this may take longer)
  • Finding identity of dead body, if prints are on record.

Finding a match: If the print already exists on the computer, a match can be found very quickly. The report would get back to the detective in an hour or two. However, the computer doesn't do what we saw in "Squeeze" with all that nice superimposing of one print on the other, then flashing a "100 % match".

The computer system the FBI is about to start using is the IAFIS (Integrated Automated Fingerprint Identification System). The FBI keeps records of over 213 million prints and receives over 40,000 requests for pritns each day. Currently, a lot of these are still on card, but more and more are being computerized. The way the new system will work is that fingerprints acquired as a result of an arrest at the city, county, or state level will be processed locally and the electronically forwarded to a state or other federal agency processing center, many of which already employ automated fingerprint identification systems. If no match is found locally, they will then be forwarded to the FBI, processed, then the results returned electronically. This whole process will, they say, take about 24 hours, though it would be possible to do in in 2 hours of it was an emergency.

This system is required as the current one doesn't meet present needs. The FBI admits that there is a backlog of about million fingerprints needing processing. On their web page, they cite several cases in which there was a two to three month delay in processing the prints of criminals. Without a computer, the only way you can guarantee on finding a match is if you have a pretty good idea of who the criminal is anyway, so know where to look.


Collecting blood from the scene Once blood is collected from the crime scene it has to be refrigerated as soon as possible. When collecting blood it is preferable to take a sample a dried area as opposed to a pool of blood, even if there is a pool available. This is because it is easier to preserve it.

Bloodstain pattern analysis The pattern of the blood splatters can be examined to find out all sorts of things, such as what type of weapon or impact occurred to cause the bloodstains present; how many times the victim was struck; where the victim was at the time the injuries were inflicted; where the assailant was during and following the assault; whether the bloodstain evidence is consistent with the medical examiner findings, whether the bloodstain evidence on the suspect and his clothing is consistent with the crime scene. Usually the bloodstain pattern expert would prefer to be at the scene, but they often have to work from pictures.

Drops falling from different heights will leave different- looking splatters. A drop falling from a low height of a few inches will leave a small cohesive circle. At greater heights, the circle will be larger and may be surrounded by a rink of smaller droplets. A droplet hitting a surface at an angle will bulge out in one direction, indicating the direction of travel.

Also, when someone is hit with a blunt object, blood adheres to the weapon. When it is pulled back ready for another blow it throws "cast off stains" in the form of an arc of blood droplets. Counting the arcs can show the number of blows. The form of the arcs can indicate the position of the assailant relative to the victim, whether the assailant was right or left handed, and what object was used.

The blood is then taken to the serology section for analysis.

The Serologist (or Criminalist) will do a typing exam within one or two days. This is an ABO typing analysis, to tell the blood type of the sample. This doesn't identify who the blood comes from, but at least allows people who don't match to be ruled out of the investigation. The test itself takes 4 or 5 hours to complete.

About 80 percent of the population are "secretors," which means that their other bodily fluids - semen, saliva,sweat etc - contain the same substances as their blood, so these can be used in the same was as if a blood sample has been found.

It is also possible to do a blood test on items of evidence that don't have visible blood on them. The lab will do a blood screen to see if there is actually blood there. This is likely on such items as knives. Even if they have been cleaned, blood can often be found on them, especially if the handle is removed.

After the typing is done and is found to match the suspect, DNA typing is done next.

The first test is PCR testing. When the police do this they usually have to send out to a private lab, costing $500-600 a sample. Three samples are sent - blood taken from the victim, blood taken from the suspect and the actual sample taken from the scene. However, the FBI would not have to pay anything for DNA analysis because they have their own lab. They won't have to pay for anything because they have everything available in house. The FBI labs will also do testing on samples sent in by other law enforcement agencies, but there is such a back-log that this takes several months.

The PCR tests compares the three samples and checks if any of them are matches. If they can do the exam with all your samples at one time it is more conducive to the testing process. As long as the samples are big enough, the test can tell if there is a match - eg if blood found at the scene matches the blood of the suspect. Where the controversy comes in is when the sample is very sparse, very little of it. Then, because there had been no standards set within the DNA field, you can have one expert say they interpret the scan to read one way and the defense will get an expert to say they see it another way.

The FBI, in answer to this dilemma, got a group of DNA experts to come up with some standards for labs that consistently do DNA analysis. They came up with TWGDAM. It's not mandatory - strictly voluntary guidelines for labs who do DNA analysis. However, if you are not adhering to TWGDAM guidelines, everyone thinks there is something you are hiding in your lab, or you would be.

DNA analysis takes at least 2 months to complete under the best circumstances. In dire emergencies it could perhaps be done faster.

The term DNA fingerprinting is never used anymore due to an outcry from Latent Print Examiners. With fingerprints we say that a latent print or whatever and an exemplar were found to be one and the same person. DNA cannot do that. They say the results of their exam in ratio form. The possibilities of another person having this same genetic bar code would be one in so many millions. On a positive match the one in so many millions is so outlandish you are left to conclude it has to be one and the same person, but they can't actually say that. That's the difference. However, the ratio, the one in so many millions or billions, is sometimes more people than are estimated to be on earth, so the chances of someone with exactly the same DNA being in the same town, near the same crime scene, is pretty remote. However, it is not possible to exclude that possibility. Only fingerprints can give an exact identification.

Hair and Fiber


Hair is virtually indestructible, except when burnt. It doesn't decay so will outlast the body and give useful information.

Hair can be examined to determine:

  • human or animal - sounds easy, but actually there are so many variations it is the work of an expert to actually say which animal the hair comes from.
  • where on the body the hair came from - hair on different parts of the body has different cross-sections.
  • race (sometimes)
  • some diseases

Also, of course, hair found at the scene can be matched against the suspect's hair, if there is one. This is usually done with hair from the head, at normal crime scenes, and pubic hair, for sexual crimes. A comparison microscope is used, and factors looked for are colour, length, diameter, distribution of pigment, granules, signs of disease, signs of dyeing etc.

Neutron Activation Analysis can study all the various elements in a hair. Everyone has various combinations of elements - eg copper, gold, sodium etc. It is claimed that the odds of two people having exactly the same concentration are a million to one.

When a hair is pulled from the head rather than falling naturally, there is a bit of blood at the root which can then be analyzed as blood is analyzed. DNA analysis can also be done on cells from hair roots


Fibers are taken from all over the crime scene to see if they match any of the suspect's clothes.

Fibers can be found from a very wide variety of materials - animal fibers of varying sorts (eg wool), vegetable fibers (eg cotton, hemp), mineral fibers (eg asbestos) and man-made fibers (eg nylon, polyester). Analysis through the microscope can usually identify the fiber, although various chemical tests are necessary for the man-made fibers.

Disputed Documents

This is used to find out the authorship of a document.

For an X-Files treatment of document analysis, see Agent Henderson in "Young at Heart".

Handwriting analysis:

(This is not graphology, where people attempt to read someone's character in their handwriting.)

Handwriting analysis is based on the principle that no two people write exactly the same way. Handwriting analysis looks at letter formations, connecting strokes between the letters, upstrokes, retraces, down strokes, spacing, baseline, curves, size, distortions, hesitations and a number of other characteristics of handwriting. The handwriting expert will usually compare a questioned sample with a sample written by the suspect and say if they are likely to be written by the same person.


Like with firearm analysis, the basic division is between "class characteristics" and "individual characteristics". If a document has been typed on an old fashioned typewriter, the individual characteristics can be found such as wear and tear on the letters, misaligned letters, letters that are depressed more or less than the others etc. At the same time the "class characteristics" such as the shape and size of the letters, the font etc can determine what model typewriter the message was printed on. Typewriter ribbons can also be read to see what has been typed there.

Laser printers and photocopiers:

These use similar methods by which a process rather like magnetism leads the black toner stuff to adhere to the paper, where it is heat sealed.

Clues can be obtained in several ways.

  • The paper can be phsyically marked by the belts, pinchers, rollers and gears that move it through a machine.
  • Toner can have unique characteristics in its chemical composition.
  • Toner can vary in the way it is placed on the paper - smoothly? In blobs?
  • Sometimes there are scratched and things on the glass or lenses that show up on the printed page.


As we saw Mulder doing in "Apocrypha," sometimes it is possible to recover writing from the impression made on the surface below.

At its simplest, the document is simply held up so the light hits it at an oblique angle, then is photographed in that light.

ESDA: This is the Electrostatic Detection Apparatus. Strangely, is you pass an electric current through paper with an impression on, the part with the impression gets charged differently. When black toner is scattered on, it sticks to the impression. However, the solutions used to get latent fingerprints makes this useless, so it must be done before looking for fingerprints.

Charred Documents:

These must be handled very carefully. The fragments can be stabilized by spraying with an acetate solution en route for the lab. At the lab, the best way to get the writing off is to sandwich the charred paper between two photographic plates and then keep in the dark for two weeks. When the plates are developed in the normal way the writing shows up as clearly as if the paper had never been burnt in the first place.

Ink samples

These can also be analyzed to find out what type of ink was used, using chromotography or spectography. There is a database of thousands of ink chromatograms to help identify the type and age.

Imprint Evidence

These can be 3-D or 2-D. An example of a 3-D imprint is a footprint sunk deep into mud. An example of a 2-D imprint is a footprint made after stepping into some paint.


Impressions of shoes in soft ground are taken with plaster, after photographing print from all sides and measuring it. In addition, the patterns on the bare human foot are as unique as a fingerprint and can be used for identification, although, unlike fingerprints, there aren't large central databases to help with this.

If it is thought that there might be a faint footprint on some types of floor, an electrostatic mat is used. A weak electrical current is passed between two sheets of black acetate, and the static charge attracts evidence to the mat.

The shoe print can reveal such things as shoe size, make of shoe, plus individual details such as sweat and tear.

Tire tracks:

Wheel base is the distance between the two front wheels and the distance between the front and rear wheels. By studying these, it is possible to compare them with databases of all makes of cars and narrow down the type of car that made the tracks.

The tread design itself can also narrow down the list of possible cars. Again, special books contain pictures of all tire tracks and the makes of car that use them.

The individual wear developed from use will show up in the impression, allowing for identification of a single car, to the exclusion of all others.

It is also possible to tell which way the car was facing and which direction they drove off in.

Tool impressions:

For example, the tool marks made by prying open a door.

These are of three types:

  • Impressions: in which a tool surface presses into a softer material
  • Scratch marks: in which a tool (such as a screwdriver) scrapes across a surface
  • Cutting: which is a combination of the above two types (as with scissors).

All types can be examined to find out both what type of tool made the impression, but also specific facts about the individual tool.

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