An inside look at the latent print with the Hennepin County Sheriff’s Forensic Science Lab

MINNEAPOLIS (FOX 9) - DNA evidence, with its high-tech luster, often grabs all the headlines. 

But latent prints, that old-fashioned staple of forensic science, are still a cornerstone of criminal investigations.

"Well, it’s definitely a combination of evidence," says Emma Verges, a Forensic Scientist with Hennepin County Sheriff. "You want to have more than just one area of support to solve the case."

Time-tested methods

What we know:

In the five-plus years that Emma’s been a Latent Print Examiner, she figures she’s looked at well over a thousand of them.

The Hennepin County Sheriff's Crime Lab averages several thousand cases each year, and at least half, she says, come through the latent print section.

While there are a variety of ways to process prints, depending on the surface or material being examined, the two most prevalent remain either using dust or a lab grade superglue.

The dust is spread using a magnetic brush, meaning there are no brush fibers that contact the item to prevent smudging the print. From there, the prints are lifted with tape, then transferred to an acetate card to be photographed or scanned.

In the glue method, lab-grade glue is vaporized in a chamber, which then sticks to the residual oils of the prints.

With the glue method, they can spray a liquid dye on the item, then take it under a laser to find additional prints that are harder to see.

Those prints, whether it be from fingertips or the palm, are also photographed and scanned into the computer to be methodically analyzed for unique markers.

"So there’s a lot of work in documenting up front," Emma explains, "and then either searching in a database to generate a candidate, or do I have a list of suspects that I can just their cards and just directly compare if it’s that person."

The human element

Big picture view:

While computers do come into play, the analysis is largely a methodical examination by the forensic scientist, looking for and marking unique characteristics of each print.

If there is a known list of suspects with fingerprints on file, it’s also done by visual examination.

"You still have to do the manual work of marking out the comparison," Emma says, to either match a print or exclude it.

Prints are also run through the Automated Biometric Identification System, or ABIS, a database which will generate potential matches.

But again, it’s up the trained eyes of a human being to examine those, too.

Using prints, either from fingers or palms or feet, has been part of the forensic process for well over 100 years.  Prints from each person are that unique.

And even in the age of DNA, print analysis can still do things that DNA cannot.

"One fun fact about latent prints is that identical twins have different fingerprints, whereas identical twins have the same DNA," Emma said.