U.S. Attorney General Eric Holder acknowledged the blunders made in the federal Fast and Furious gun-tracking operation in an appearance before Congress, but some Republicans remained unappeased and continued to call for his resignation.

During a packed Senate Judiciary Committee hearing Tuesday, Holder said the sting conducted by the Bureau of Alcohol, Tobacco, Firearms and Explosives' Phoenix chapter, which allowed assault rifles and other weapons to be illegally purchased from Arizona gun shops and smuggled to Mexican drug traffickers in 2009 and 2010, was unacceptable and acknowledged that its repercussions would be felt for years.

The idea behind Fast and Furious was to trace the weapons to powerful drug traffickers in Mexico, but once it got underway ATF agents realized they had no dependable way to keep track of the guns, which eventually began appearing at crime scenes on both sides of the border.

The operation was "flawed in its concept and flawed in its execution," Holder said, acknowledging that Fast and Furious weapons will likely continue to appear in the future at crime scenes in both the United States and Mexico.

It was Holder's first appearance before Congress since top Republicans probing Fast and Furious accused the attorney general of misleading them in earlier testimony about when he first learned about the details of the sting.

Holder has been the target of a probe by Sen. Charles Grassley (R-Iowa) and Rep. Darrell Issa, (R-Calif.), who said last month in a joint statement that the attorney general received at least five memos in mid-2010 that described the ATF's "gun-walking" strategy.

Holder had testified in a congressional hearing in May of this year that he had learned about the program just a few weeks prior.

In Tuesday's hearing, Holder said he first learned about Fast and Furious at the beginning of the year and should have said in May that his knowledge of the sting dated back a couple of months rather than weeks.

During the hearing, Grassley also pointed to a letter dated Feb. 4 in which the Justice Department said the operation did not involve gun-walking and that "ATF makes every effort to interdict weapons that have been purchased illegally and prevent their transportation to Mexico."

Holder admitted that the letter contained erroneous information from ATF's Phoenix office but refused to accept blame.

He said he was continuing to receive information from ATF personnel as late as March that assured him that no gun-walking - as efforts to track rather than interdict weapons purchased by suspected arms smugglers is known - was taking place.

Pressured by Sen. John Cornyn (R-Texas), Holder acknowledged that he is ultimately responsible for what happens within the Justice Department, which oversees ATF, but he added that he cannot be expected to be aware of the day-to-day management of every operation.

Referring to a timeline presented by Cornyn, Holder said he did not learn about the gun-walking tactic used in Fast & Furious until the true nature of the controversial operation became public early this year.

The attorney general's remarks, however, did not placate Republicans in Congress.

In statements to reporters, Cornyn criticized Holder for not being informed nor demonstrating curiosity about the details of the operation, although he stopped short of calling for the attorney general's resignation, as dozens of House Republicans have done.

Grassley, meanwhile, told reporters that he wants to gather all the evidence and get to the bottom of Fast and Furious, adding that Holder must resign is he is found to be responsible for the program.

The lawmakers also are seeking more information about a similar - but smaller - gun-tracking operation in 2007 during President George W. Bush's tenure.

The Justice Department's Office of the Inspector General is due to craft a report on the details of the Fast and Furious operation, although no date has been set for its release.