Crawling Fingerprints?

Fingerprints have long been one of the cornerstones of forensic crime scene analysis. From their early use in the late 19th century, to their first role in a murder conviction in New York in 1902, to their standard use as we know it today, fingerprints and their analysis have become crucial tools for investigators in their pursuit of criminal justice. Where some other techniques have come into question—such as bite mark analysis—fingerprints have always been considered reliable. There are surfaces that prove problematic, or visualization techniques may not be powerful enough, but the concept of the ability to match a single individual to a single print has never been shaken.

Fingerprints are, in essence, biological traces left by individuals marking their contact with a surface. As Matt Lowell put it in TWO PARTS BLOODY MURDER, a fingerprint is “an organic slurry of amino acids and fats with some inorganic compounds mixed in” we leave behind when we touch a surface. Patent prints are left when a substance is transferred by a finger, leaving a visible print behind i.e. ink or paint. Latent prints are invisible impressions of the slurry Matt describes that need to be processed to be visualized, and are the majority of the prints law enforcement deals with. Fingerprinting can be a difficult endeavor as a pristine, complete print is rarely deposited. Instead, prints overlap, only consist of a partial impression, smear or smudge, or are a mixture of different individuals. Adding to that is the composition of the surface the print is on, the age of the print, and the type of processing involved. It’s a complicated process, but when it works well, the answer is definitive.

A paper was recently released discussing how latent prints change over time, and how they change shape and can actually migrate over certain surfaces. Over time, any fingerprint will lose water content and the bulk of the print ridges will decrease. But it was the placement and positioning of those ridges that was the key to this study.

Some surfaces do not maintain fingerprints well—prints on certain types of plastics will disappear in about four days, where a similar print on glass will remain for months. Porous surfaces such as paper and wood absorb some of the oils and are excellent matrices for locking the print into place. But some materials actually allow the print structure to change as the ridges decrease in height, but increase in width, while the space between the ridges increases. In essence, the print spreads laterally, migrating outward, covering up to 140% of the original surface in just over a week. However, given sufficient time—up to eight weeks—the print will contract, eventually only taking up 69% of the original size.

How does this kind of migration affect a print in a criminal investigation? The authors suggest that this kind of print expansion and contraction could be responsible for a number of the print mismatches that still occur today. They also suggest that if a timetable of migration could be determined, digitized prints could be reverse-aged back to their original structure, which would allow for direct comparison to fresh suspect prints. The authors have suggested this technique would be particularly useful on new polymer banknotes which are already proving a challenge for traditional fingerprinting methods. This technique could prove to be beneficial as it could help investigators overcome a significant problem with fingerprints—a timeline. The presence of a print linked to an individual is a crucial piece of information. But knowing when that print was deposited—yesterday, last week, or last month—could be the difference between a suspect who was in the room at the time of a murder, or a week before, when the victim was still hale and healthy.

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