Mayor Bloomberg has joined the attack against the nation’s embattled attorney general, saying Alberto Gonzales’s Department of Justice has become “complicit” in the “disgrace” of an effort to further restrict gun trace information.

The mayor is directing his rage at a proposal that would require law enforcement officers to “certify” that any gun trace information they request will be used only for a criminal investigation or prosecution, a plan he says the justice department once disputed. The mayor says that if the information is used for any other reason, such as tracking broader gun sales patterns, law enforcement officers could be prosecuted.

“We’re at a loss to explain the justice department’s flip-flop on this issue, or how the nation’s top law enforcement agency could advocate criminalizing good police work,” Mr. Bloomberg wrote in a letter to Mr. Gonzales.

The Department of Justice initially said yesterday that the measure it supports “says nothing about criminal prosecutions of police officers,” but later in the day dropped that from its statement, noting that someone could technically be subject to prosecution if they lied on their certification.

A spokesman for the agency, Erik Ablin, then said: “The provision on certification was not included with the goal of criminal prosecutions of law enforcement; rather it is a way for the department to ensure that it is releasing trace information consistent with longstanding” practice of the Bureau of Alcohol, Firearms, and Tobacco.

Mr. Bloomberg’s letter to Mr. Gonzales illustrates his take-no-prisoners approach to cracking down on illegal guns. Mr. Gonzales, who has come under fire in recent weeks for his role in the firings of U.S. attorneys, is the latest in a long line to get a taste of Mr. Bloomberg’s wrath.

In return, the mayor has been moved to the top of the hit list of Second Amendment proponents, the ATF, and the gun lobby, and at least four members have dropped out of his national coalition of Mayors Against Illegal Guns.

Still, the mayor has been leading the charge to repeal the so-called Tiahrt Amendment, a 2003 provision that keeps gun statistics out of the hands of municipalities such as New York City that want to map out trends or prosecute civil cases against gun dealers.

Last month, the mayor’s coalition funded a television advertisement targeting Rep. Todd Tiahrt, the Republican of Kansas whom the amendment is named for. Mr. Tiahrt responded in the Wichita Eagle that the advertisement was “blatantly misleading” and arguing that the amendment does not prevent authorities from getting the information they need. He also noted that the Fraternal Order of Police fully supports the measure.

Mr. Bloomberg’s criminal justice coordinator, John Feinblatt, disputed the justice department’s characterization that the latest proposal does not differ from past measures. He said it’s no mistake that the measure now suddenly includes the word “certified.”

“If this was just a reflection of standing ATF policy, why would you add the word certified? It’s a word that pops off the page to any lawyer that reads it,” Mr. Feinblatt said.

He cited a portion of the budget proposal that says if a law enforcement agent violates the certification provision, he could face up to five years in federal prison.

Mr. Bloomberg’s letter to Mr. Gonzales says the attorney general pledged to reconsider his support for the Tiahrt Amendment during a meeting at City Hall last year. But he said a “stunning about-face” by the justice department made the measure “even worse.”

In a May 2006 letter provided by City Hall, Assistant Attorney General William Moschella told a congressman that penalties similar to those now being proposed could have a “chilling effect” and “adverse consequences for law enforcement operations and officer safety.”

The city in the last year has filed lawsuits against 27 out-of-state gun dealers as part of an undercover sting operation it’s conducted. It, therefore, has an interest in ensuring that it can use gun trace data for its own litigation.

Mr. Bloomberg said that while the Tiahrt Amendment allows authorities to trace information for one gun at a time it imposes restrictions that “rob us of our best opportunity to identify the trends and patterns that define the illegal gun black market.”