Touch or trace DNA, being invisible to the naked eye, is also easily contaminated. Anyone who enters and leaves the crime scene will leave DNA behind, according to the basic laws of transference spelled out in Locard's exchange principle. "For this reason, good crime scene investigators show up armed with gloves, booties, masks and so forth to reduce the possibility of contamination," Lacks says. "This is critical with touch or trace DNA because you cannot see it. You might destroy it because you don't know it's there."
Those at the scene must ward against contamination as much as they possibly can.
Take for instance an Orange County crime scene where a DNA sample matched a profile from a different crime scene — a case-to-case hit. Officials originally suspected the same individual committed these crimes. However, the investigative process revealed a detective at the scene of both crimes had inadvertently left his DNA behind.
"Officers and deputies must recognize their actions could cause them to leave their own DNA, and take the appropriate precautions," Gialamas says, noting it's crucial that agencies establish a database of elimination samples, or standards, from those who regularly process crime scenes.
Because talking spreads an individual's DNA across an area, Orange County requires officials to converse outside of evidence collection sites. Besides donning protective gear, responding officers in this agency also are instructed to secure the location, cordon the area off and then determine a path through the scene that reduces the possibility of evidence contamination.
Finding the right guy
"We've been extremely successful in retrieving DNA — we're able to recover DNA in 68 percent of our burglary cases," says Gialamas. "That's pretty phenomenal because it means in most cases we come across, we're going to find DNA."
The advantage is DNA is prevalent at the crime scene. The disadvantage, however, is it may not be the offender's. "There's no doubt in my mind you're going to find DNA at any crime scene — locating DNA is not the issue," Gialamas emphasizes. "It's finding the DNA pointing to the perpetrator that is the problem."
"DNA is not the end all to be all," he cautions.
Finding "foreign" DNA at the crime scene, does not necessarily mean it's from the perpetrator. In one cold hit involving a burglary at an Orange County residence, the point of entry was a broken window. By the time crime scene investigators arrived to process the scene, the homeowner had replaced the broken glass. "On the windowsill was a tiny drop of blood, which we expected to be the suspect's," Gialamas says. "We had every reason to believe this was the case, given the crime."
Forensic analysts ran elimination samples on household residents and from the investigative agency, eliminating anyone who had legitimate contact with the residence. They then submitted the profile to the state database and got a cold hit. Investigators contacted the suspect and learned he was the contractor who'd installed the new window.
"This is a good example of forensic science doing exactly what it is expected to do," Gialamas says. "We found a sample, got a DNA profile, received a hit, but it wasn't the complete answer. You had to rely on the total investigation. In this case, the window installer had a prior background, which is why he had a record in the system."
Gialamas warns everyone in the criminal justice community not to put DNA on a pedestal where they believe it will answer all their questions. "In any criminal investigative process," he says, "it is the totality of the investigation — the quality of the work of the investigator, the crime scene responder, the crime lab personnel — that leads to any success."