The Washington Post obtained serial numbers for nearly 9,000 Smith & Wesson and Colt revolvers and Glock semiautomatic handguns that the District of Columbia's Metropolitan Police Department traded from 1988 to 1998. Those serial numbers were then entered into a computer and compared to a database of about 800,000 crime guns traced by the Bureau of Alcohol, Tobacco and Firearms from 1988 to 1998. A database of more than 37,000 guns traced by the Virginia State Police from 1993 to July 1999 was used for further matching.

The Post also compared about 4,600 serial numbers of guns formerly used by the Virginia State Police and 1,650 from Baltimore County police.

Altogether, The Post identified 136 guns that were linked to crimes: 107 from D.C. police, 18 from Virginia State Police and 11 from Baltimore County police.

The analysis by Post Database Editor Sarah Cohen was able to capture only a portion of former police guns used in crimes, because the ATF has trouble tracking such guns.

For example, from 1994 to 1998, the database traced more than 2,800 guns back to a "government and/or law enforcement agency," a figure that has been widely used in news reports as the total of police guns used in crimes. But the figure is misleading, The Post found, because it includes a number of guns that had been fired on and off duty by police officers. The police agencies had asked ATF to trace the guns in routine investigations into officers' use of force.

Undercounting appears to be an even bigger problem in the ATF's system.

Fewer than half the guns used in crimes in 1998 generated an ATF trace. Of those that were traced, only about 40 percent were successfully tracked to the first purchaser.

The reasons: Criminals often remove serial numbers from their guns, dealers may have gone out of business, or the guns may be considered too old.

And even when ATF successfully traces a gun, its database often fails to identify it as a former police gun. Of the District police guns The Post found involved in crimes in the ATF database, only five of 107 were identified in the database as former government weapons.

ATF also makes it difficult for the public to identify former police guns used in crimes. It took The Post eight months to obtain serial number data from ATF. The agency continues to refuse to provide most of the information needed to trace a gun's path, such as the name of the dealer that sold the gun and the police agency that investigated the crime.