WASHINGTON (AP) — Within hours of the Navy Yard shootings, the FBI was tracing the gunman's recent purchase of a shotgun, sending agents to the shop in northern Virginia where he bought it — and leaving the Bureau of Alcohol, Tobacco, Firearms and Explosives out of the loop.

The surprising snub between top U.S. law enforcement agencies comes as the ATF struggles to show its relevance in Washington.

The ATF is the federal agency in charge of tracing guns used in crimes, including the military-style semi-automatic rifle used in a rampage at a Connecticut school last year, a similar rifle used in the deadly shooting at a Colorado movie theater and a handgun used to kill six people and critically wound a congresswoman, among others, outside a grocery store in Tucson, Ariz.