The federal agency that regulates firearms says that the city of Chicago crossed a line by issuing a report providing granular detail about guns used in crimes, including the names of stores that have sold a disproportionate share of those weapons. The Chicago Trace Report, released on Sunday, runs “contrary to federal law,” Mary Markos, a spokesperson for the Bureau of Alcohol, Tobacco, Firearms and Explosives, said in an email, and constitutes “a prohibited use of the data.”

The law in question is a budget rider first passed by Congress in 2003 known as the Tiahrt Amendment. It prohibits the ATF from using any federal funding to disclose trace data — information that helps police identify where a firearm was sold and who bought it — except in aggregate statistical reports. It does not address whether local authorities are also subject to its restrictions.

A representative for Mayor Rahm Emanuel of Chicago contested the ATF’s conclusion that the report was unlawful, and suggested the bureau was motivated by the post-election political climate.

“This is just classic,” Adam Collins, a spokesman for the mayor, said in a statement. “The Trump administration is arguing it would be better to hide the facts on the deadly effects of gun trafficking than partner on a solution? Well, burying your head in the sand won’t change the truth and it won’t solve the problem.”

The ATF has long discouraged law enforcement agencies from releasing trace data. But in October of 2016, an ATF spokesman gave a substantially different answer in response to a question from this news organization about whether an earlier Chicago trace report violated the law.

Brian Garner, in an emailed statement to The Trace, said that, while the agency “would have advised against publicizing such data,” meaning Chicago’s 2014 report, “the requesting agency owns trace data for firearms they submit. They can do with the data whatever they want.”

Markos now says her colleague “misspoke.” Garner has since left the bureau’s public information office. He is now the agent in charge of the bureau’s office in Austin, Texas.

Emily Hampsten, an aide to Senator Dick Durbin of Illinois, who has taken a keen interest in law enforcement use of ATF tracing resources and opposes the Tiahrt Amendment, said in an email Wednesday that “our understanding is consistent with Mr. Garner’s understanding.”

Whether or not municipalities are legally permitted under Tiahrt to release the information they obtain from the ATF has “always been a grey area,” said Mike Bouchard, a retired ATF official who served in the bureau for 20 years through 2007 and now directs the ATF Association, an alumni group.

Chelsea Parsons, an expert in gun policy at the Democratic Party-linked Center for American Progress, said she believes that the ATF is likely taking a harder line about trace date to avoid retribution from a pro-gun Republican dominated Congress. “To be clear, the Tiahrt rider only applies to ATF and any release of trace data by the local police agency does not implicate this federal law.”

On Wednesday, the National Shooting Sports Foundation, a gun-industry trade group, issued a press release slamming the Chicago report. The NSSF claimed the report “starts from a purposely misleading premise and represents what we have heard before as a political narrative from the administration of Mayor Rahm Emanuel,” which seeks “to again blame the firearms industry for problems that require great political will to address.“ The NSSF did not claim the release was illegal, however.

While the scope of the Tiahrt Amendment is a matter of dispute, the ATF sets its own additional conditions on local law enforcement access to its electronic tracing request system, requiring agencies to sign “memorandums of understanding” that spell out how agencies may use the data.

An undated sample memorandum of understanding on the ATF’s website says that agencies that sign an eTrace agreement “may not disclose law enforcement sensitive firearms trace information generated pursuant to this MOU to a third party without the consent of ATF.”

It was not immediately clear if the document signed by the Chicago Police Department contained such a clause, though ATF sources said similar provisions are common.

In her email, Markos said the agreements make clear that the information is not for public consumption. “Through the eTrace MOU, parties agree that no data will be publicly disseminated, and that it is law enforcement sensitive information as it relates to criminal investigations,” she said.

She said the ATF was not consulted by Chicago before the report was released. She declined to comment on whether Chicago’s release would affect the city’s access to the tracing system in the future.

Chicago’s trace report is an outlier. No other major city has made public its trace information in at least a decade. Sgt. John Chafee of the Atlanta Police Department confirmed in an email that his city considers trace data to be the property of the ATF, not the local police. He said the local police force does not retain trace information it gets from the federal government.

The only other notable release of store-specific trace data, aside from Chicago’s earlier report, came in 2006, when the New York Police Department and the city’s Mayor’s Office used trace data to perform their own investigation into gun dealers from far out of the state. The investigation became the basis for 15 lawsuits against stores in five states.

More frequently, cities grant outside academics access to raw trace data for research purposes. This week, Philip Cook, a professor at Duke University, presented a paper that correlated records of guns traced with the Federal Bureau of Investigation’s database of stolen weapons, to analyze the role that theft plays in supplying the criminal weapon market. Last month, Anthony Braga of Northeastern University published an analysis of Boston’s criminal gun market, using the Boston Police Department’s comprehensive trace database, which largely consists of data provided by ATF traces.

Markos declined to comment on whether the ATF now considers granting such access to researchers illegal.

Timothy McLaughlin contributed reporting to this story