The FBI is no longer analyzing gunshot residue in its investigations, a blow to the once-highly regarded evidence used to suggest that a suspected criminal had fired a weapon.
Lawyers, scientists and law enforcement officials across the country said that they were astonished by the decision and that it could sound the death knell for the evidence. It also could become a weapon for defense lawyers in pending cases and in efforts to overturn convictions.
"If the premiere forensic science organization in the world isn't using gunshot residue, that certainly raises some questions about it," said Timothy S. Brooke of the American Society for Testing and Materials, which sets the policies used by many police crime labs.
Special Agent Ann Todd, spokeswoman for the FBI Laboratory, said the change was communicated electronically to FBI field offices on March 15, though it has not been widely publicized.
Todd said the FBI stopped analyzing gunshot residue because of a shift in priorities, not because of a lack of confidence in the science. The lab had performed the analysis for decades, but in recent years had been receiving fewer than 10 requests per year, she said. The agency decided its resources were better used in "areas that directly relate to fighting terrorism," she said.
But the FBI's abandonment of the evidence followed a closed-door summit last June to discuss the agency's gunshot residue policies and subsequent contamination tests at the agency's crime lab in Quantico, Va.
The resulting contamination study, obtained by the Sun, documents the presence of hundreds of particles consistent with gunshot residue in several areas of the lab. Such contamination could endanger the validity of analyses in criminal cases.
This marks the second time in a year that the FBI has distanced itself from forensic evidence. In September, the agency decided to stop making comparative bullet lead analyses, a four-decades-old technique that purports to link a fired bullet with a particular box of bullets.
FBI officials cited the agency's new focus on terrorism as its reason, but about that time, the National Academy of Sciences released a report calling comparative bullet lead analysis unreliable.
Even as jurors -- exposed to television shows such as "CSI: Crime Scene Investigation" -- are increasingly hungry for forensic evidence, skepticism has grown about the way in which it is used in court. A key problem has been that all trace evidence, including gunshot residue, can be presented to jurors with a false degree of certainty.
Gunshot residue is made up of the microscopic particles that explode from a gun when it is fired. The particles can be collected from suspects' hands, analyzed and used as evidence in court.
Called "GSR," the particles float like ash and never disintegrate. There can be a danger that surfaces -- such as a police officer's hands or laboratory work tables -- can become contaminated and then contaminate fresh samples.
Frederic Whitehurst, a former FBI crime lab employee who became a whistle-blower in 1997 when he exposed questionable lab practices, said the science behind gunshot residue analysis is basically sound. It's the unavoidable contamination, he said, that has been a pervasive problem.
A.J. Schwoeble, director of forensic science at Pennsylvania-based RJ Lee Group, which services 500 law enforcement agencies and crime labs, said contamination concerns can be overcome.
Police departments and crime labs simply must take precautions, he said, such as having personnel wear gloves at a crime scene, storing samples in sealed vials and frequently testing the lab.