The Latent Print and Footwear unit examines items using a variety of physical and chemical methods to visualize "latent" prints
and footwear impressions. When prints or impressions of value for identification are located, they are compared with
standard or "known" impressions.
Friction Ridge Impressions
Complex physical patterns are formed by the ridged skin on the fingers and palms of the hands (commonly referred to as fingerprints and palm prints) and soles of the feet and toes (footprints). Given a sufficient area, these patterns can be identified (individualized) to one person. Fingerprints, (and palm prints and footprints), are unique to each individual. Even identical twins do not have identical fingerprints, palm prints, or footprints.
While the overall appearence of monozygotic (identical) twins' fingerprints is often similar (a and c),
they differ in the fine details (the locations of ridge starting and stopping).
Which is a finger and which is a toe? Complex physical patterns are formed by the ridged skin
on the toes and soles of the feet as well as the fingers and palms. The patterns, while often appearing
roughly similar are always markedly different in the fine details regardless of source.
Crime scene friction ridge prints ("latent" prints) are chance impressions made under uncontrolled conditions;
as such, they are often fragmentary and of poor quality. Latent prints are deposited when a person touches a surface
with oils or perspiration on his fingers or palms. Criminals sometimes attempt to avoid leaving latent prints by
wearing gloves or handling items gingerly. Even a small movement while handling an item can cause the image deposited
to be blurred and unusable.
"Known" impressions are obtained by inking a person's fingers and palms and rolling them on a white card. They can also be obtained using "live-scan" computer equipment. Using live-scan, a person's fingers are scanned one-by-one and digitally recorded by computer. No ink is required, and the computer prints out a standard 10-finger card.
An identification is made when there is sufficient correspondence in the unique characteristics to cause the examiner to conclude that the known and latent prints were made by the same finger/palm/toe/foot.
Latent prints are made visible using a variety of chemical and physical methods. Nonporous surfaces such as glass are examined or "processed" using methods including cyanoacrylate ester ("superglue") fuming, luminescence (laser), dye staining, and traditional brush and powder. Porous surfaces such as paper may be processed using chemical methods such as diazafluoren-one, ninhydrin, and physical developer. The exact processing sequence will depend upon a number of variables including surface type, environmental factors, the length of time elapsed since suspected handling, and the potentially destructive effects of each technique that would preclude subsequent examination by other methods or for other types of evidence.
Lasers are used to visualize fingerprints. Items are "superglued" and the superglue residue is stained with a
luminescent dye. The item is then examined in a darkened room using the laser. Goggles are worn to filter out the colors
of light the laser produces directly. Latent prints visualized with the laser are not visible to the naked eye in
ordinary room light, and must be photographed.
Once latent prints are located, they are compared with inked or live-scan exemplars. Until recently if no suspect was named, no comparison was possible; it was physically impossible to, essentially at random, manually compare recovered prints against a large collection of fingerprint cards. This limitation was overcome with the introduction of Automated Fingerprint Identification Systems (AFIS) in the 1970's.
Conceptually, AFIS works as follows: Inked fingerprints are scanned and converted to computer-readable representations of the fingerprints. Those representations are stored in a computer database. When a fingerprint is recovered from a crime, it is scanned into the computer, converted to computer-readable form, and searched against the database. The computer then ranks the known prints according to the closeness of agreement to the crime scene print, by generating a list of "candidates." The computer cannot make an identification. Its sole function is to decrease the number of suspects the examiner must manually compare.
Many systems (the Wisconsin system included) allow unmatched crime scene fingerprints to be stored in an Unsolved Latent database. These prints are then automatically searched by the computer against each new fingerprint card entered into the system in the future. Even if the maker of a latent print is not in the database at the time the latent print is first searched, when he is arrested and entered later--even years later--the match can be made. To date, identifications have been made in Wisconsin as much as five years after the crime.
Criminals often don masks or gloves to conceal their identity. However, they must walk to and from the scene. Each footstrike carries with it the possibility of identification of their footwear as the sole source of that mark.
Footwear identifications are less common than for fingerprints for several reasons. First, such marks are not as commonly sought or recovered by crime scene investigators. Second, the clarity of reproduction required for conclusive individualization is not often present. Third, individualization is only possible when random, "accidental" characteristics have been imparted to the sole of the shoe that are reproduced in the impression. When this accidental detail is present in sufficient quantity and quality, it is possible to individualize a print to a specific shoe, to the exclusion of all other possible sources.
Through wear, the soles of footwear acquire imperfections such as cuts, gouges, and embedded objects (a).
If the transfer is clear enough, these imperfections may be reproduced in each footstrike (b) and in comparison
standards prepared by the examiner (c).
Even when conclusive individualization is not present, it is still possible to limit the number of shoes that could have made a mark. In a recent Wisconsin case, the manufacturer was able to estimate that only approximately 25 pairs of shoes were sold in Wisconsin that could have made a particular pair of impressions (out of a total production of about 600,000 pairs). Given that there are approximately 1 billion pairs of shoes sold each year in the U.S. (and approximately 20 million pairs sold in Wisconsin), the value of this "class-only" evidence assumes greater significance.