Washington ? After handguns with bullets that can pierce body armor started showing up on the streets of New Jersey, police and lawmakers wanted to know where the weapons were coming from.

But when Sen. Frank Lautenberg, D-N.J., asked the federal agency in charge of keeping gun trace data for help last year, he was told the information was off limits.

Tiahrt amendment

That response is guided by a federal provision known as the Tiahrt amendment. The measure prevents the Bureau of Alcohol, Tobacco, Firearms and Explosives from sharing certain types of gun data with cities or local law enforcement authorities.

The amendment, first passed in 2003 by Rep. Todd Tiahrt, R-Kan., and typically attached to a major spending bill each year, has been the source of an increasingly bitter debate over how to curb gun violence and a nasty feud between the Kansas congressman and a group of mayors led by New York City Mayor Michael Bloomberg.

“The current Tiahrt amendment makes police officers do their job with a blindfold on,” said John Feinblatt, Bloomberg’s criminal justice coordinator. “What you want to know is, for my jurisdiction, are there a small number of dealers who keep supplying crime guns to my city? And you can’t get that information.”

Mayors’ No. 1 issue

Bloomberg’s nationwide coalition of Mayors Against Illegal Guns has made repeal of the Tiahrt amendment its number one issue this year. With seed funding provided by Bloomberg, a billionaire, the group of more than 200 mayors last week began airing television ads in several media markets – including Tiahrt’s Wichita district – calling for repeal of the amendment.

Tiahrt claims the ads are misleading and argues that his amendment prevents disclosure of sensitive information about pending cases that could jeopardize active investigations. He also asserts that the ATF itself has misunderstood the language of his amendment, leading the agency to withhold too much information.

“I think that personal attack is something that’s very upsetting to me and very unjustified and a great mischaracterization of all my efforts here,” Tiahrt said.

Two TV stations in Wichita declined to air the ads, saying they could not verify the claims.

Under the Tiahrt amendment, local law enforcement agencies can obtain gun trace data if it relates to a specific criminal investigation or prosecution. But many police chiefs, mayors and their supporters in Congress say cities need the ATF’s broader data to help fight crime.

“The Tiahrt amendment deprives us of critical information in the fight against violent crime – information like trends in illegal gun trafficking, the types of guns that find their way into the hands of criminals, and which gun shops provided the bulk of guns found in crimes,” Lautenberg said. “We need to be able to identify where guns come from to combat gun violence and protect our communities.”

Tiahrt sees a political motive at work. Before his amendment placed restrictions on trace data, the information had been used in lawsuits and other proceedings to revoke the licenses of gun dealers that sell large numbers of guns later used in crimes.

“They say they want the data to prevent crime, but they have no plan to use that data to prevent crime,” Tiahrt said. “I think perhaps what they want it for is to move a political agenda forward. A gun control agenda possibly.”

In fact, New York City has sued more than two dozen gun dealers around the country, alleging they sold firearms illegally to undercover private investigators conducting a sting for the city. Officials claim hundreds of weapons used in New York City crimes came from the dealers.

‘Mayoral vigilantism’

While the movement to repeal the Tiahrt amendment has the backing of 10 law enforcement groups – including the International Association of Chiefs of Police – Tiahrt points out that his measure also has powerful support from the nation’s largest law enforcement organization, the Fraternal Order of Police.

“This is like mayoral vigilantism,” said Jim Pasco, the FOP’s executive director. “It’s not their law to enforce. They go out like a bull in a China shop and wander around outside their jurisdictions trying to make civil cases. It’s absurd.”

Another powerful supporter of limiting access to gun trace data is the National Rifle Association, which says the Tiahrt amendment protects the privacy of gun owners. At the time his amendment first passed in 2003, Tiahrt acknowledged it had the blessing of the NRA.