Tracing a pistol’s path from gun shop to crime scene is the first step toward “locking the back door” on illegal trafficking, President Clinton said Saturday as he expanded a firearms tracing program to 10 additional cities.

“Make no mistake: Gun traffickers are funneling guns to lawless youth,” he said in his weekly radio address. “We know how they operate, and we intend to shut them down.”

Since the president’s Youth Crime Gun Interdiction Initiative started in 17 cities last year, law enforcement agencies have tracked 37,000 crime guns to their sources - almost twice as many as the year before, according to Treasury Department statistics.

But the number of gun murders by juveniles has quadrupled since the mid-1980s and Clinton said it would take an expanded gun tracing program, the continuation of background checks on gun buyers and the cooperation of Congress to stem that.

Clinton said he was “disappointed” that a Senate committee recently rejected his proposal to require federal firearms dealers to provide child safety locks.

The president also urged law enforcement agencies to continue doing background checks on prospective gun buyers, despite the Supreme Court’s decision last month to strike down the part of the Brady Law that mandates such checks.

Background checks “keep criminals from getting guns through the front door of a gun shop,” Clinton said. And with the expanded tracing program, “we’re turning our attention to locking the back door, too.”

The tracing program found that four out of 10 crime guns recovered by police came from juveniles and the most commonly used weapon for people under 24 was a semiautomatic pistol. The program also found most crime guns came from out of state.

The tracing program was started in 17 cities: Seattle; Atlanta; Baltimore; Birmingham, Ala.; Boston; Bridgeport, Conn.; Cleveland; Inglewood, Calif.; Jersey City, N.J.; Memphis, Tenn.; Milwaukee; New York; Richmond, Va.; St. Louis; Salinas, Calif.; San Antonio, Texas; and Washington.

The program will be expanded to 10 more cities: Los Angeles; Philadelphia; Chicago; Detroit; Houston; Miami; Tucson, Ariz.; Minneapolis; Gary, Ind.; and Cincinnati.