Mayor Bob O'Connor recently joined other mayors in a national campaign aimed at reducing gun violence.

"There are too many illegal guns, not just in our city, but throughout the country," he said. "This is a national problem and must be addressed at a national level."

What Tiahrt amendments include Here are some of the restrictions on the Bureau of Alcohol, Tobacco, Firearms and Explosives, commonly called the Tiahrt amendments, that were attached to funding legislation in 2003, 2004, last year and this year and which some members of Congress now want to make permanent: Trace data. No funding is available for the release of ATF gun trace information to anyone except law enforcement, and then only to police who are conducting a bona fide crime investigation limited to their own jurisdiction. The information essentially has become classified and is immune from Freedom of Information Act requests and civil subpoenas. ATF also cannot provide trace information to a third-party police department, even with the permission of the department that requested the data. Sales. ATF cannot release any information about multiple gun sales by a dealer to a single buyer, one of several "trafficking indicators" the agency uses to scrutinize problem dealers. ATF also cannot require dealers to provide information on sales of used guns, another potential indicator. Background checks. The FBI's National Instant Criminal Background Check System used to keep gun sale records for 90 days to make sure that no mistakes were made, such as allowing a felon to buy a gun. That time frame was reduced under the Tiahrt amendments to 24 hours. In 2002, the Government Accountability Office warned that such a short time frame would make it difficult for authorities to retrieve guns from people who should not have been approved to buy them. The gun lobby said keeping background check information would amount to a de facto national gun registry, which the organization fears might be used one day to confiscate guns. Officially, ATF says it has adjusted to the 24-hour limit. Disclaimer. If ATF publishes any studies on tracing, it now must include a disclaimer saying that it can't draw conclusions from the data. The ATF used to analyze and draw conclusions about gun trafficking patterns in its reports.

But that's not easy when the federal agency that monitors gun trafficking can't share all it knows because of restrictions approved by Congress that are intended, in part, to shield gun dealers from lawsuits.

Mayors are now banding together to oppose new efforts in Congress to permanently limit access to gun trace data from the Bureau of Alcohol, Tobacco, Firearms and Explosives.

Police in Pittsburgh and around the country recover thousands of guns used in crimes every year and try to track their origins through ATF's National Tracing Center.

ATF used to analyze that data routinely and report on trends, such as the fact that a tiny minority of gun dealers sell the majority of guns used in crimes.

But these days, that kind of intelligence is secret. Pittsburgh police, for example, won't reveal anything about traces on guns used in crimes in the city.

The reason is that a series of recent gun lobby-backed riders to the bills that fund ATF prohibit the agency from disclosing trace information to anyone but the police department that requests it. That means it's off limits to researchers, mayors, police in neighboring communities and, apparently, members of Congress.

A new bill would make those restrictions and others permanent. It also would, according to the Justice Department, "effectively criminalize" the sharing of trace data among police departments.

In a recent letter to a House committee, the department said the bill, HR 5005, would have a "chilling effect" on police. Gun control advocates and some federal agents say it could keep law enforcement from "connecting the dots" in battling the black market gun trade.

"We're not serious about guns in this country," said Joseph Vince, the former head of ATF's crime gun analysis branch who now runs Crime Gun Solutions, a consulting company. "It's the NRA [National Rifle Association]. They wield the money. No one can give me another reason."

The appropriations riders were inserted by Rep. Todd Tiahrt, R-Kan., an ally of the NRA. The new bill was introduced by Rep. Lamar Smith, R-Texas, who also is backed by the gun lobby.

Mr. Tiahrt didn't return a phone call requesting comment. But Mr. Smith said the bill was supposed to keep the data from being used by litigants for "political purposes," meaning suits against the gun industry.

The NRA says it supports Mr. Smith's bill "out of concern for gun owners' privacy and the confidentiality of law enforcement records."

Sheree Mixell, spokeswoman for ATF, said the current restrictions under the Tiahrt amendments have not hindered the agency's ability to share information with police. She said the agency is not allowed to comment on pending legislation.

The Justice Department, though, has recommended removing a key part of the legislation that would limit data-sharing between police jurisdictions.

Assistant Attorney General William Moschella said gangs and gun-runners don't care about borders.

"Absent information about trace data spanning jurisdictions," he wrote, "links and information about the criminal activity between the jurisdictions may be difficult to develop in many cases."

While HR 5005 wends its way through the House, other bills have been introduced to reverse the Tiahrt restrictions. But they're not likely to go anywhere in a Republican-controlled Congress.

Classified data
Gun trace analysis is critical in exposing patterns, which lawmakers can use to set policy.

One example is the assault weapons ban which was allowed to expire in 2004. To figure out if more AK-47s and similar weapons will turn up in shootings, politicians would have to assess gun traces.

But the law says they can't.

"I don't think Congress realized they were limiting even their own access to the data," said Elizabeth Haile, a staff attorney for the Brady Center to Prevent Gun Violence.

Such confusion abounds on this issue.

Some mayors have expressed outrage that trace information is restricted. Police departments also have been trying to figure out what they can and can't do with their own trace data.

"Most of the questions reflect a basic fundamental misunderstanding of these relatively new restrictions," wrote Charles Houser, chief of ATF's tracing center, in a recent police newsletter.

Under the Tiahrt amendments, passed every year since 2003, the contents of ATF's Firearms Trace System have become classified, he wrote.

The agency can't disclose data to anyone except police, and then only if officers have a bona fide investigation in their jurisdiction. The data also is immune from Freedom of Information Act requests and civil subpoenas.

But there's nothing in the law about what police can do with the reports, which are marked "law enforcement only." Police have interpreted that to mean they can't release any data, which is Pittsburgh's stance.

There also has been confusion about whether police can share the data with other police. ATF says they can as long as they are working together on a bona fide case.

The gun lobby has not been alone in seeking to keep trace data under wraps. Many in law enforcement, such as former Attorney General John Ashcroft, opposed its release because of the "risk to ongoing investigations."

Advocates for open information concede that much of the data is detailed and should be kept confidential to protect informants and agents. But they say releasing reports based on aggregate data has never hurt anyone.

In a letter to Mr. Smith, Mr. Vince said that, in his three decades with ATF, "I have never encountered or even heard of a case where the release of trace information to law enforcement, or to the public, for that matter, has compromised or impeded a criminal investigation or prosecution."

Bad apples
Interstate gun intelligence is not as important in Pittsburgh as it is in some places because most crime guns recovered here are stolen locally.

But in the nation's biggest cities, most guns come from states with less restrictive gun-purchase laws. Those cities want all the information they can get. New York has led the way in filing suits against manufacturers and dealers who might be letting guns get into the hands of criminals.

The Brady Center and its allies say the restrictions on ATF have been designed to limit discovery in the most important suit, a pending federal case filed by New York against the gun industry in 2000.

The NRA and its allies say the information is being sought to stack the case against dealers.

A federal judge has admitted the trace data into evidence for certain years, although it will remain sealed from the public.

Yet much of the same information was once widely available in ATF reports issued under former President Bill Clinton. The most controversial one showed that about 57 percent of crime guns nationwide could be traced to about 1 percent of gun dealers.

U.S. Sen. Chuck Schumer, D-N.Y., raised the ire of the NRA in 1999 when he used that data to identify 137 dealers by name as the "very worst" because they each had sold at least 50 guns used in crimes.

The gun lobby argued that a high number of crime gun traces doesn't necessarily mean a dealer is doing anything wrong. For one thing, the lobbyists said, guns can be tracked back only to the initial sale. If a gun exchanges hands after that, a dealer wouldn't necessarily know about it.

The industry said that some guns bought at stores with high-volume sales inevitably would end up being used in crimes.

"If guns are used in crime by the customer of a dealer, that plainly has nothing to do with whether the dealer acted unlawfully," according to a statement issued by Mr. Smith.

Yet one ATF report before the information clampdown refuted that claim. It said some dealers had a disproportionate number of crime gun traces that couldn't be explained by high-volume sales.

In addition, some 86 percent of gun dealers had no crime guns traced to them at all. In fact, some of those legitimate dealers have urged ATF to go after the "bad apples" in their ranks.

'The iron pipeline'
ATF uses gun traces and other "red flag" indicators, such as a high number of lost guns, to determine if a dealer warrants closer scrutiny.

One of the most notorious dealers is Trader Sports in San Leandro, Calif., which was identified by the Brady Center as the nation's second-largest supplier of guns used in crimes.

After finding that the store could not account for 1,723 guns, ATF revoked its license after a five-year investigation.

The store doesn't sell guns anymore, although its owner has filed suit to restore his license.

But critics argue the Justice Department isn't doing enough to go after such dealers, especially those who fuel the "iron pipeline" by which guns travel from states with lax laws to those with stricter laws, typically from the South to the North.

A commonly cited example of how the network operates took place in Charleston, W.Va.

In 2000, New Jersey cocaine dealer James Gray recruited Tammi Songer, a cab driver with a clean record, and paid her to be his straw purchaser. She paid $4,000 for a dozen handguns at Will's Jewelry and Loan Co., although Mr. Gray selected the guns and carried them out in full view of the clerk.

A year later, one of the guns ended up in the hands of a felon who used it to wound two police officers in Orange, N.J.

The officers sued the gun maker and the pawn shop. A judge threw out the suit against the manufacturer, a decision hailed by the gun industry, but the pawn shop settled for $1 million and agreed to stop high-volume sales.

Mr. Gray and Ms. Songer went to federal prison.

The Justice Department has long prosecuted straw purchasers in such cases. But many out-of-state guns still end up on the streets of Boston, New York, Chicago and elsewhere.

The iron pipeline is what sparked New York's latest suit, filed last month after Mayor Michael Bloomberg sent private investigators to make straw purchases at suspect gun dealers across the country.

The mayor said he took the action because ATF was "asleep at the switch."

Some who agree say the current federal enforcement approach under Project Safe Neighborhoods is like combating drugs by arresting users but not suppliers.

Federal prosecutors routinely indict people caught carrying guns illegally. In Pittsburgh, the U.S. attorney's office has won convictions in hundreds of such cases over the past five years.

But dealers are harder to prosecute because the burden of proof is high: Prosecutors must show that a dealer knowingly sold a gun to someone who can't have one. The result, critics say, is that the supply is never choked off.

"It's like bailing the Monongahela with a bucket," Mr. Vince said. "The way you do it is the way New York did it."

ATF doesn't agree. Wally Nelson, the agency's recently retired deputy assistant, ripped the New York sting last week as a "headline-grabbing stunt" by Mr. Bloomberg.

Ms. Mixell, the ATF spokeswoman, said she couldn't comment on Mr. Vince's criticism.

But she said federal gun trafficking convictions had increased in recent years and that the agency "has a number of active investigations nationwide."