Latent Prints

 

The Latent Print section evaluates and analyzes evidence for the presence of fingerprints, palm prints and footprints.  Any one of these types of prints can be formed by the impression of the friction ridges (raised portion) present on skin.  The impression left can be made by the natural secretions from sweat glands in friction ridge skin, referred to as a latent print. A latent print is a crime scene print.  Normally it is invisible, but not always.  Processing allows the print to be visualized.  Or they can be made by ink or other materials transferred from the peaks of the friction ridge skin to a relatively smooth surface, referred to as a patent print.

 

Two types of evidence are typically submitted:  lift-cards that have latent prints taken from a crime-scene, and objects to be processed at the laboratory for the presence of latent prints.  The section receives a variety of types of evidence for latent print processing.  Some examples include:  paper, (e.g. checks, notebooks, letters, money); plastic, (e.g. plastic bags, credit cards, bottles); glass, (e.g. bottles, mirrors); metal, (e.g. guns, knives, cash registers).

 

The composition of an object will determine which processing technique will be utilized to develop latent prints.  There are a variety of chemical and physical methods that can be used including superglue fuming, luminescence (laser), dye staining, powdering, ninhydrin and physical developer.  Many of these techniques can be used in tandem, for example, a plastic bag would first be superglue fumed then processed with Rhodamime dye. 

Fingerprint Record

After the analyst has processed the evidence to preserve and develop prints, the items are photographed.  The ridge detail observed in the photographs is then examined to determine if it is of value for comparison.  This evaluation process is also conducted on submitted lift-cards.  The first step is to compare any prints that are of value to the victimís record fingerprints, or to anyone who had legitimate access to the crime scene.  Sixty to seventy percent of latent prints examined are identified as belonging to the victim.  The second step is to then compare any remaining unidentified prints to the suspectís fingerprint record.  The third step, if there are any remaining unidentified fingerprints, is to enter them into the Automated Fingerprint Identification System (AFIS).

 

The Illinois State AFIS system contains over 3 million sets of fingerprints from arrestees, job applicants, police officers and civil service employees in the State of Illinois.  Prints that are entered from crime scenes where the suspect is unknown are also in the system and are referred to as unidentified latents (UL).

 

A search of the AFIS system will produce a list of persons whose fingerprint resembles the entered print.  The analyst will then obtain a copy of the original fingerprint record to conduct the comparison.  An identification is made when the examiner determines that there are enough characteristics that correspond in both the questioned and known print that allows the examiner to conclude they were made by the same person.