From Ammoland:

U.S. Sen. Chuck Grassley (R-Iowa) released a series of whistleblower documents that raise the ghosts of the Bureau of Alcohol, Tobacco, Firearms and Explosives’ (ATF) ill-fated Operation Fast & Furious. That was the Department of Justice (DOJ) operation that allowed firearms to be illegally smuggled across the U.S. border to Mexico, except once they crossed, they were never tracked.

One of those illegally-trafficked firearms was used to murder U.S. Border Patrol Agent Brian Terry.