Crime Library: Criminal Minds and Methods

LA Forensics: The Signature Murders

Processing for Prints

The photographers went through first to photograph everything in the position in which it lay. Then after a walk-through to see the layout of the scene, a pair of latent print specialists went through the apartment to find and lift fingerprints. As with any inside scene, there would be plenty of prints from people who resided there. Some would be smudged or partial and others would be clear. What they hoped was that the intruder had come without gloves and had touched something in such a way that he would have left clear prints from more than one finger. That was the best case scenario.

Examining Fingerprints
Examining Fingerprints

They were able to lift around 30 fingerprints, which they carefully packed for analysis. The deceased's prints would be rolled at the autopsy for comparison, and detectives would collect prints from anyone else who might have been in the apartment.

For about a century, this has been the best evidence for the identification of an offender. Our fingertips are covered with ridges and valleys, some of which make continuous lines, some of which stop, some of which divide, and some of which make other kinds of formations like arches, pockets and dots. These patterns are classified into four basic groups, with subgroups, making eight overall pattern types. Comparisons are made by finding a similarity on several points between the lifted print (questioned sample) and those taken from a person (known sample). The more points of similarity there are the better, but only one dissimilar point is sufficient to negate a match between the known and questioned samples.

Ridged skin leaves impressions thanks to tiny sweat glands hidden within. Sweat mixes with amino acids, creating a substance that leaves a residue. Touching any surface transfers the perspiration present in the ridge and valley patterns, leaving an impression of the minutiae. Or the person might touch something that clings to the skin, such as cooking spray or ink, and thereby leave an impression. Depending on how much surface there is and how many points of similarity, the impression can be sufficient to provide a lead. If the impression is not visible but can be made so with a specific technique, it's called a latent print.

Making a latent print visible depends on the type of material on which it was left. The more irregular or absorbent the surface, the more difficult it is to lift a good print, although many advances have made in this regard.

Initially, prints were developed on nonporous surfaces with fine, gray-black dusting powder, applied with a soft brush; this technique is still practiced today. The excess powder is carefully blown off, leaving a clear impression from the powder that adheres to the oils. The print is then ready to be photographed and lifted with a special tape. It's then placed onto a special print card for preservation.

Besides powder, there are other methods, such as the use of chemicals, for surfaces like paper and cardboard. Then digital imaging was developed, along with Superglue fuming.

Portable light sources can help to locate prints, especially if used with fluorescent chemicals and powders. Holding light at an angle over a surface provides oblique lighting that can show the presence of the natural residues of a print.

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