Meanwhile, law enforcement agents say gun traffickers tend to run small operations, creating further challenges.
"There is not one big warehouse in Tennessee that we can shut down and solve the problem," said U.S. Attorney Rod J. Rosenstein. "They are coming from a lot of small sources."
Also, the people who buy and sell guns on the illicit market tend to be sophisticated and hard to catch, said Daniel Webster, a co-director of the Johns Hopkins Center for Gun Policy and Research. His work has shown that there is broad awareness among criminals that police do ballistic traces on weapons, which makes them reluctant to purchase a used gun or buy from anyone they don't trust. "The thought that runs through their mind is, 'Where has that gun been?'" he said.
Webster has been advising Baltimore's Police Department to focus on suppressing demand for weapons. "The most cost-effective way of reducing gun violence is to focus on illegal possession of guns and making that a very risky thing to do," he said.
Baltimore police have been pursuing that approach, particularly in the past year under the city's new commissioner. Officers from specialized units have flooded the high-crime Eastern and Western districts with instructions to look for known criminals with guns. This year, the murder and shooting rates have fallen by 30 percent citywide. Much of the gain comes from a steep decline in murders in the Eastern and Western districts.
But city officials say that the efforts to go after suppliers are worthwhile, too.
"If almost half of our crime guns is coming from outside of the state, we need to be able to see who the main culprits are," said Sheryl Goldstein, the head of the Mayor's Office of Criminal Justice. "We need to be able to work with our regional partners to combat that."
A comparison of the sources of guns used in crimes shows that Maryland and New York have eight of the top 10 states in common, so Goldstein is working with her counterpart in New York City to build a database so that police departments in both cities can share data about guns they recover. She hopes that other cities in the region will link into the database, enabling detectives to mine for patterns about the origins of illegal guns.
Maryland's top supplier states last year were Virginia, North Carolina and Pennsylvania.
The federal government already tracks national and regional gun-trafficking trends, but a congressional restriction prevents federal agents from giving local police data about cases outside their borders.
