The shooter walks up the driveway and pulls the trigger. In a split second, the pistol’s firing pin makes contact with the primer, a pressure-sensitive device that ignites the gunpowder in the ammunition. The buildup of the resulting gases propels the bullet out of its casing toward a target leaving behind burned gunpowder residue and trace amounts of metals including lead, copper, and brass inside the barrel. Vaporized particles escape from the breach and muzzle; they cool and reform as microscopic particles. The particles fall on to shoes, clothing, fixtures and furniture, but most importantly onto the target and the shooter. That’s why investigators seek to determine whether suspects in a shooting show evidence of gunshot residue (GSR). But it may not be as easy to assess guilt as once thought.
But despite the limitations of GSR evidence, Trimpe writes that it need not be abandoned. He notes that research has provided guidelines that can lead to increased confidence in GSR interpretation and sample integrity.
To get the most accurate samples, Trimpe writes that they should be taken at the crime scene whenever possible and before transporting the subject in a car. Although a Colorado Bureau of Investigation study showed that an entire building could be contaminated, Trimpe focuses on the proximity of the testing to firearms areas; he advises GSR testing should never be near the firearms area of a facility, and samples should be airtight at all times, except immediately before and after being tested.
Historically, criticism of GSR examinations includes the fact that they can’t conclusively prove someone was the shooter. While, as Trimpe writes, “Numerous population studies have shown that GSR is not normally found on the average person,” the presence of gunshot residue is nonetheless circumstantial. And studies have shown that the potential for contamination from law enforcement personnel, vehicles, and testing labs exists.
In May 2006, one year after a majority of GSR experts at a symposium had agreed that a handcuffed person could be contaminated by GSR in the backseat of a police vehicle, The Baltimore Sun
reported that the FBI was discontinuing GSR examinations.
GSR was found in 14 of 26 law enforcement vehicles in a study by CBI. Another five vehicles had trace amounts of at least one of the three types of particles (Most primers in the United States are made of lead, barium, and antimony).
The Baltimore Sun also reported that a study of a 500,000-square-foot lab in Quantico with independent venting in each room and corridor still showed GSR on desks, railings, and door handles throughout the building.
GSR examination in the news:
– Last week, The News and Observer reported
that charges were dropped against a woman accused of shooting her stepdad in the face and leaving him for dead in their driveway. The nightgown she was wearing was covered in gunshot residue, but there was no residue on her hands.
– Researchers in Texas develop a more sensitive test
that can detect GSR in particles 15 times smaller than the diameter of a human hair – even in lead-free ammunition.
– The FBI discontinues
use of GSR in investigations.
- At a symposium in Quantico, Michael Martinez of the Bexar County Criminal Investigation Laboratory said of 100 handcuffed subjects in the custody of local law enforcement, 16 percent had trace elements of GSR on their hands
. He concluded "the particles were transferred from a law enforcement officer, an inanimate object, or the back of the law enforcement vehicle in which the subjects had been transported." GSR experts agreed that a handcuffed person could be contaminated by GSR in the backseat of a police vehicle.
– A study published in the Journal of Forensic Sciences
says brake linings in vehicles can produce similar residue to GSR but can sometimes be distinguished from GSR because of high iron content.