Holder Testifies: Fast and Furious Guns Will Be Showing Up 'For Years to Come

Attorney General Eric Holder said Thursday guns from the botched Fast and Furious operation will continue to show up at U.S. and Mexican crime scenes "for years to come."

"It is an unfortunate reality that we will continue to feel the effects of this flawed operation for years to come," Holder said during his testimony before the House Judiciary Committee Thursday. "Guns lost during this operation will continue to show up at crime scenes on both sides of the border."

Holder's testimony follows his appearance before the Senate Judiciary Committee last month, during which he admitted the failed program -- where weapons sold to Mexican drug cartels were not tracked, resulting in the deaths of hundreds including a U.S. Border Patrol agent -- was "flawed."

"As I have repeatedly stated, allowing guns to walk – whether in this administration or in the prior one – is wholly unacceptable.   The use of this misguided tactic is inexcusable. And it must never happen again," Holder said Thursday.

Facing harsh questioning from lawmakers, Holder again denied any top department officials were aware of the "inappropriate tactics" used in the operation until they were made public. Holder himself has changed the timeline on when he first knew about the program, saying in May it was only within the last few weeks before amending that statement in November.

Rep. Darrell Issa, R-Calif., chair of the House Oversight and Government Reform Committee and a leader in the investigation, said he found Holder's claims "implausible" and demanded to know why those involved at the Justice Department have not been fired.

"The president has said he has full confidence in this attorney general," Issa said. "I have no confidence in a president who has full confidence in an attorney general who has in fact not terminated or dealt with individuals, including key lieutenants who from the very beginning had some knowledge and long before [U.S. Border Patrol agent] Brian Terry was gunned down, knew enough to stop this program."

"Mr. Attorney General, the blame must go to your desk and you must today take the real responsibility," Issa said.

(Watch Issa's statement below. Comments about Obama beginning at the 3:45 mark.)

Rep. Jim Sensenbrenner (R-Wis.) said "heads should roll" and raised the possibility of impeachment, though without specifying exactly whom he had in mind.

"If you don't get to the bottom of this," there is only one alternative, and "it's called impeachment," Sensenbrenner said. More than 50 lawmakers have called for Holder's resignation.

The Associated Press contributed to this report.