An expert in forensic science testified Friday that Jose Ines Garcia Zarate has exactly one particle of gunshot residue on his hands – enough for the prosecution to claim as evidence that he fired a pistol, but little enough to give the defense grounds to argue that it was inconclusive.
Linda Abuan, who works at the San Francisco crime lab, explained to the jury that every time a firearm is discharged, it releases hundreds of microscopic particles, some only 1/40th the thickness of a human hair.
A distinctive particle, which contains antimony, barium, and lead, is characteristic of that residue, she said. And the discovery of any amount on a person’s hands indicates that the person “fired a gun, was close when a gun was fired, or touched something with GSR on it.”
She went through the testing procedure, which involves a scanning electron microscope searching swabs taken from a suspect’s hands. In this case, the microscope found one particle that didn’t appear to have the characteristics of GSR.
But, following standard procedures, Abuan went back and manually examined that particle and decided that it fit the profile.
It’s not a quantitative test, she said: “We are just looking to see if it is present or not.” So one microscopic particle can trigger a positive result.
On cross-examination, Matt Gonzalez, representing Zarate, asked her if everyone in the forensic world agreed that the presence of one particle was enough to demonstrate that the person had fired a weapon. In Baltimore, he said, the Police Department considers five particles the minimum for a positive identification. Abuan acknowledged that there is no commonly accepted threshold.
In fact, Gonzalez pointed out, researches in Los Angeles found GSR in 45 of 50 samples from the back seat of police cars. The FBI lab in Quantico has found it on desks and railings.
Abuan said that GSR can be spread easily by contact; she compared it to talcum powder. It can last for weeks or months, she said; it doesn’t degrade and remains where it was until is it brushed or washed away.
Judge Samuel Feng has been very strict about limiting questions in corss-examination, the courts don’t like speculation. We are under no such rules, so we can raise the question:
If, as the defense argues, Zarate was holding the gun, possibly wrapped in a shirt, when it discharged, then tossed it, possibly still wrapped in cloth, into the Bay, he might have had only a limited amount of GSR on his hand.
On the other hand, some people fire a weapon and get almost no GSR.
I’m not sure how much that testimony proved, particularly since the defense isn’t denying that Zarate was close to the weapon when it fired.