WASHINGTON — The head of the Bureau of Alcohol, Tobacco, Firearms and Explosives says that in at least one instance during a now-controversial gun-running investigation his agents did not intercept high-powered weapons when they could and should have.

Kenneth Melson's acknowledgement is the first by any senior ATF leader that confirms some of the criticism that Republicans on Capitol Hill have been leveling at Operation Fast and Furious. The concerns have resulted in congressional hearings and an inquiry by the Justice Department's inspector general.

The operation was designed to track small-time gun buyers up to major weapons traffickers along the Southwest border. Critics estimate that 1,800 guns targeted in the operation are unaccounted for and that about two-thirds of those probably are in Mexico. Republican lawmakers released excerpts from Melson's closed-door testimony.