The FBI is no longer analyzing gunshot residue in its investigations, a blow to 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 they were astonished by the decision and said it could mean the end of using such evidence.
It also could become a weapon for defense attorneys in pending cases and in efforts to overturn convictions.
"If the premier 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, including Baltimore's.
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 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 its gunshot residue policies and subsequent 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 jeopardize criminal cases because it casts doubt on the origin of the residue in a sample.
This marks the second time in a year that the FBI has distanced itself from forensic evidence. In September, the agency announced it had stopped making comparative bullet lead analyses, a four-decades-old technique that purports to link a fired bullet with a particular box of bullets.
The FBI cited concerns about costs of maintaining the equipment and the resources necessary to do the examination as reasons for its decision, while saying it "still firmly supports the scientific foundation" of the analysis. But that change came on the heels of a National Academy of Sciences report that called comparative bullet lead analysis unreliable.
"In my experience, forensic labs only abandon techniques they've been using for decades when they realize that what they're doing is junk science," said Michele Nethercott, co-chairman of the forensics committee of the National Association of Criminal Defense Lawyers and a public defender in Baltimore.
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. And some once-powerful prosecution tools, such as microscopic hair analysis and bite-mark comparisons, have been discredited in recent years.
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 - from a police officer's hands to laboratory work tables - can become contaminated and then contaminate fresh samples.
An ASTM committee also is in the process of reviewing its policy about gunshot residue, which was last revised in 1995, said Brooke, director of ASTM's technical committee operations. FBI agents, along with state and local police officers, scientists and academics, are on the review committee.
On a local level, Baltimore prosecutors said they have been using the evidence sparingly in recent months, though the city Police Department's crime lab turns over about 400 gunshot residue tests each year.
Reports last year in The Sun raised questions about the gunshot residue analysis conducted by the city crime lab.
A 2001 study found gunshot residue in testing areas that were supposed to be clean, according to internal police documents. Police officials have said they solved that problem by moving the testing area away from firing ranges.
And unlike dozens of other agencies, including the Maryland State Police and, until March, the FBI, Baltimore's crime lab counts a two-element particle as gunshot residue. Most agencies use a higher standard: considering only a three-element particle to be gunshot residue.
Matt Jablow, spokesman for the city Police Department, said the city lab will revisit its policies after the ASTM has completed its review of the gunshot residue standard. ASTM's current policy includes the two-element particle as gunshot residue.