San Diego Police Department

SDPD Crime Lab Using DNA From Ammunition to Link Suspects to Crimes

A significant number of violent crimes involve handguns, and sometimes, the only evidence left is the cartridge casing.

DNA profiling has proven invaluable to investigators with the San Diego Police Department, as advancing computer software connects evidence to suspects.

But until recently, shell casings left at crime scenes were not tested for DNA.

"Previously, other agencies, including ourselves, either thought there was so little DNA left on ammunition after it was fired, or that the act of firing heated up the casing to a temperature at which the DNA was destroyed," said Patrick O'Donnell, supervising criminalist at the SDPD crime lab.

But as the sensitivity of DNA analysis improved, SDPD criminalists reevaluated that assumption.

In 2015, the crime lab tested nearly 1,000 cartridge casings from previous investigations and recovered usable DNA profiles in about 25 percent of the cases.

O'Donnell and another criminalist, Shawn Montpetit, authored their conclusion of the 9-month study in Forensic Science International.

Subsequently, all ammunition left at crime scenes are tested for DNA.

A significant number of violent crimes involve handguns, and sometimes, the only evidence left is the cartridge casing, O'Donnell explained.

"A lot of times this is just like a puzzle," he added. "You place one piece in the puzzle, and that makes it easier to place the next piece."

Even more amazing advances are on the horizon.

It's only a matter of time that DNA profiling will be able to give investigators the kind of suspect description they currently obtain with a witness interview and a composite sketch.

"An emerging trend in DNA that I think is promising is the ability to physically predict the characteristics of the individual that was present at the crime scene," said O'Donnell.

Criminalists would be able to take blood evidence and assess a series of genes to determine a person's eye color, facial shape, and height, and eventually, whether that person has freckles or is predisposed to baldness.

Such technology could be valuable in serial homicides and sex assaults where evidence is limited and the public is endangered, O'Donnell explained.

O'Donnell said another revolution in DNA analysis is the increasingly sophisticated software that can perform hundreds of thousands of calculations within a matter of seconds.

That data is especially helpful when numerous DNA profiles are present at a crime scene: for example, at a convenience store robbery or when several people have handled a handgun.

While DNA has become the gold standard of forensic analysis, today, about half the time, it's still fingerprint evidence that helps investigators identify a suspect.

Only a tiny portion of a person's fingerprint is needed for an identification. Computer software can also enhance blurry and low contrast fingerprints.

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