"Touch DNA" has expanded the types of items that can be considered physical evidence. Everything from gun and knife handles, to steering wheels, basically any physical object the suspect touched can be used to try to develop a DNA profile.
As a person crimes detective for the last 10 years, I've had a lot of experience with DNA. It's been amazing to see the technology and techniques grow and develop to the point where we can get DNA from swabbing a bicycle handlebar or from the inside brim of a baseball hat.
Just a few months ago, I received lab results from our state lab reference a sexual assault I'd been assigned to work. The victim reported that the unknown suspect used both of his hands to pull her underwear down. I spoke to the laboratory agent about the details of the case. The lab agreed to process the underwear for "touch DNA," concentrating on the parts of the underwear the suspect would have touched.
We got a hit. The swabs from the victim's underwear contained a partial DNA profile that presumably belongs to the suspect. Amazing. This unknown suspect left behind enough DNA from just briefly touching the sides of the victim's underwear to provide a statistically significant partial DNA profile.
Strengths and Weaknesses
Despite the amazing nature of Touch DNA technology, CBI Laboratory Agent Schleicher is quick to point out that agencies need to have realistic expectations and should understand that "touch DNA" has the same strengths and issues as other types of forensic DNA results.
First, "touch DNA," like standard DNA and fingerprints for that matter, doesn't tell the investigator when the DNA was left on the evidence. It could have been left an hour ago or a week ago. Of course the investigator may be able to narrow down the time range based on certain facts of the particular case.
An example is a suspect's baseball cap left behind at a murder scene. The lab swabs the inside and outside of the hat. A DNA profile from two individuals is developed. Even if there's more DNA from one contributor than the other, you still can't say who was wearing the hat at the time of the murder.
Also, "touch DNA" is so sensitive that it's possible to pick up background DNA. For example, if a shirt is made by hand, then someone has touched the shirt even before it's packaged and sold. It's possible "touch DNA" could liberate these skin cells from the evidence, even though this person has nothing to do with the investigation.
And of course, "touch DNA" doesn't tell the investigator how the DNA made its way onto an item. It doesn't provide the culpable mental state of the individual that committed the crime.
It's up to the lawyers to argue whether the "touch DNA" is the result of a casual contact, or from the suspect forcefully grabbing the victim's shirt. The bottom line is that good police work is necessary to piece together the events surrounding the commission of the crime.
Schleicher wants agencies to be aware of the power and limitations of the technology. Agencies need to clearly understand the questions they want answered. Why is this piece of evidence important? What will it tell me about the commission of the crime? Law enforcement agencies just need to be prepared for the answers, even if the answers aren't what they were expecting.
David Spraggs is a major crimes detective and a certified bomb tech for the Boulder (Colo.) Police Department. He is a member of the POLICE Advisory Board and a frequent contributor.