On December 14 of last year, Border Patrol agent Brian Terry was fatally wounded in a shootout. Now, it seems, the Bureau of Alcohol, Tobacco, Firearms, and Explosives is refusing to divulge information about how Terry’s killers acquired the guns they used.

Jaime Avila was arrested in connection with the incident the day after it happened. Avila, it turns out, was a suspected weapons trafficker — whose gun purchases had been monitored by the ATF for a year, and who allegedly bought two weapons used in the shooting from a dealer who was cooperating with the ATF through the Project Gunrunner program. According to Sen. Chuck Grassley (R., Iowa), the ranking Republican on the Judiciary Committee, ATF whistleblowers told his staff that the agency allowed the sale. In addition, when one dealer involved in Project Gunrunner said he wanted to stop selling to traffickers, he was told to keep doing so, and to report the sales to the ATF, according to the whistleblowers.

When Grassley requested an explanation from the ATF, the Justice Department denied that the ATF knowingly allowed illegal purchases, but the ATF itself has thus far failed to brief Grassley’s staff. There may be a good reason for the agency’s failure to stop the sale. Either way, it owes Brian Terry’s family — and the American people — an explanation.