Agents lost track of hundreds of guns that flowed south to Mexico, where many were recovered at crime scenes. Two such guns were found in the United States at the scene of the killing of border agent Brian Terry.
Holder told the panel that he became aware of the gun-walking tactic at the same time as the public and that he found out "about the same time" that guns found at the scene of Terry's death were part of Operation Fast and Furious.
Holder has faced off repeatedly with Rep. Darrell Issa of California and other Republicans in recent months over his handling of the aborted firearms investigation. Issa's committee has prepared a contempt citation against Holder but not voted on it yet, applying pressure for more documents on the operation.
Holder said the Justice Department has cooperated fully with congressional investigators and turned over 7,600 pages of material to Congress about the operation.
"Look, I don't want to hear about the 7,600," snapped Issa, chairman of the House Oversight and Government Reform Committee that is investigating the operation.
At Thursday's hearing, House Democrats placed on record letters urging Issa not to pursue Holder's contempt citation. The letters included notes from Philadelphia Police Commissioner Charles H. Ramsey, the National Organization of Black Law Enforcement Executives, the survivors of the mass shooting in Tucson that wounded former Rep. Gabrielle Giffords, and the National Action Network led by the Rev. Al Sharpton.