A 2003 law pushed by the gun industry limits the information shared by federal agents and shields gun shops from public scrutiny, but ProPublica was able to identify the store that sold the gun used in the shooting of a Chicago police officer.

Two decades ago, federal and local law enforcement routinely identified the source of guns used in crimes to members of the media or anyone else who inquired.

That changed in 2003 when Congress, bowing to pressure from the gun industry, approved legislation known as the Tiahrt amendment, named after a former Rep. Todd Tiahrt, R-Kan., a gun rights champion. The amendment bars police and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives from disclosing any information they uncover during gun-tracing investigations, including the names of retailers.