IT IS IRONIC that a city that banned handguns nearly 25 years ago and that has paid thousands of dollars to get 3,100 guns out of circulation has actively put nearly three times as many weapons back in the market during the past 10 years. That is a key finding of a Post investigation that linked guns formerly used by D.C. police to 107 crimes -- including homicides -- committed throughout the country.

Nationally, the number of police guns involved in crimes could run into the thousands, according to writers Barbara Vobejda, David Ottaway and Sarah Cohen. Guns are ending up in the wrong hands, in part because the District and many other police departments trade used guns for lower prices on new firearms.

In 1989, for example, D.C. police traded in more than 3,600 Smith and Wesson and 600 Colt revolvers and bought 4,300 new Glock 9mm semiautomatic pistols. Five years later, the department traded in 4,646 used Glocks and obtained 4,000 new weapons at no cost. Police in Maryland and Virginia have engaged in similar transactions. Fairfax County opted out of the practice in September after The Post reported that one of Fairfax County's seized weapons was involved in a drug crime in Richmond. County leaders voted to destroy former police weapons and all seized firearms.

Confronted this week with findings that old weapons from his department had turned up in the hands of criminals, D.C. Police Chief Charles Ramsey also said that he would end old weapons swaps and that if the police department purchases new guns, it would "destroy those currently in our possession."

Police departments still trading guns should follow the District and Fairfax lead. Law enforcement ought not be in the business of fueling the criminal gun market. Yet that is precisely what happens when police guns turned in to gun makers end up in gun shows where unlicensed sellers peddle them to buyers without background checks. The practice borders on police insanity.