Attorney General Eric Holder suggested Thursday that weapons lost during the course of the failed "Fast and Furious" gunrunning operation will continue to show up at crime scenes in the U.S. and Mexico "for years to come."

Holder, in testimony on Capitol Hill that comes as the congressional investigation into the program expands, decried the "gun-walking" tactic used in the operation as "inexcusable" and "wholly unacceptable." But a day after an influential senator called for the resignation of one of Holder's top deputies over the scandal, Holder denied department leaders played any role in the crafting of "Fast and Furious."

He continued to assert that top Justice officials were not told about the "inappropriate tactics" until they were made public.

Still, the top law enforcement official in the country conceded that, as a result of "Fast and Furious," guns lost by the Bureau of Alcohol, Tobacco, Firearms and Explosives remain in the hands of criminals.

"Although the department has taken steps to ensure that such tactics are never used again, it is an unfortunate reality that we will continue to feel the effects of this flawed operation for years to come," he said. "Guns lost during this operation will continue to show up at crime scenes on both sides of the border."

Holder faced the ire of skeptical lawmakers who are demanding accountability. Wisconsin Republican Rep. Jim Sensenbrenner told Holder Thursday that he considers impeachment on the table if Holder doesn't figure out who's responsible.

"You know, the thing is, is that if we don't get to the bottom of this -- and that requires your assistance on that -- there is only one alternative that Congress has, and it's called impeachment," he said. "It is an expensive and messy affair. And I don't want to go this far."

Congress has been investigating "Fast and Furious" for nearly a year. Scrutiny of the program intensified after guns from the program were found at the scene of Border Patrol agent Brian Terry's murder.

Republican lawmakers in recent weeks have complained about inconsistencies in the Justice Department's public accounting of the program over the past year. Sen. Charles Grassley, R-Iowa, the ranking Republican on the Senate Judiciary Committee, on Wednesday pointed to those alleged inconsistencies in calling for the resignation of Lanny Breuer, chief of the department's criminal division. Grassley accused Breuer of withholding information about gunwalking tactics used in a Bush administration-era program known as Wide Receiver, and of not being forthcoming about whether he saw a Justice letter to Congress in February that inaccurately claimed ATF was not letting illegal guns walk across the U.S.-Mexico border.

Though Breuer denies seeing the memo, Grassley pointed to emails that show he was sent a draft of the letter.

The Justice Department is standing by Breuer, and Holder testified Thursday that department heads were not aware of the program early on.

"The documents produced to date also belie the remarkable notion that this operation was conceived by department leaders, as some have claimed," Holder said. "It is my understanding that department leaders were not informed about the inappropriate tactics employed in this operation until those tactics were made public and, as is customary, turned to those with supervisory responsibility over the operation in an effort to learn the facts."

Holder said such a program "must never happen again," but effectively urged lawmakers to move on -- and tackle the broader issue of the flow of firearms into Mexico.

"We cannot afford to allow the tragic mistakes of 'Operation Fast and Furious' to become a political sideshow or a series of media opportunities," he said. "Instead, we must move forward and recommit ourselves to our shared public safety obligations."

He used the occasion to prod Congress to support efforts to give the Justice Department broader legal tools to track firearms purchases.

But Republican lawmakers continued to put pressure on Holder about how he's responding to the operation.

"This project was failed and flawed from the beginning," said Rep. Darrell Issa, R-Calif., questioning why Holder has not terminated the "many people involved" with the program.

"Mr. Attorney General, the blame must go to your desk," Issa said.

Holder later said that he's "ultimately responsible" for actions in the department, but stressed the actions he's taken to get to the bottom of the operation once he learned about it.

Despite the controversy over the inaccurate February letter from Justice, Holder also stated: "Nobody in the Justice Department has lied."