Washington (CNN) -- Attorney General Eric Holder said Tuesday that the controversial tactic that allowed illegal guns to be smuggled to Mexico "should never have happened, and it must never happen again."
In testimony to the Senate Judiciary Committee, Holder hit hard at the Bureau of Alcohol, Tobacco, Firearms and Explosives practice that has tainted his tenure at the Justice Department and led to some Republican calls for him to resign.
"I want to be clear: Any instance of so-called 'gun walking' is unacceptable," Holder said of weapons smuggling, later adding: "This operation was flawed in its concept, and flawed in its execution."
Holder acknowledged what critics have been saying about the long-term consequences of "gun walking."
"Unfortunately we will feel its effects for years to come, as guns that were lost during this operation continue to show up at crime scenes both here and in Mexico," Holder said. "We are losing the battle to stop the flow of illegal guns to Mexico."
Holder defended his own actions, reminding the panel he called for the inspector general to examine the so-called Operation Fast and Furious and issued a directive that "gun-alking" was illegal and should never be repeated. The resulting report may be concluded before the end of the year.
However, Holder stopped short of laying blame for the flawed operation.
Last week, his assistant attorney general, Lanny Breuer, expressed "regret" for his failure to alert Holder about a similar earlier operation called "Wide Receiver" which had employed similar ATF "gun-walking" tactics during the Bush administration.
At the hearing Tuesday, Republican Sen. Charles Grassley of Iowa said, "It doesn't matter how many laws we pass if those responsible for enforcing them refuse to do their duty -- as was the case in Fast and Furious."
Grassley pressed Holder at the hearing about who had leaked a sensitive document to the press regarding a whistle-blower who had come forward with allegations of gun-walking. Grassley asked who was being held accountable.
Holder refused to answer, but several hours later Grassley said Justice had provided him a document in a classified setting.
Grassley revealed that former U.S. Attorney Dennis Burke, who resigned from his post, had leaked the document to the press.
Still, the senator said, he is not satisfied. "The Justice Department should not be allowed to continue to scapegoating the one person who has resigned," Grassley said, promising to pursue the matter further.
Despite the disagreements, Holder pressed for congressional cooperation. He made a plea to the committee for strengthening the authority of the ATF to detect and disrupt illegal gun trafficking, and urged full funding for the ATF to combat gun-running.
He added: "I am determined to ensure that our shared concerns about Operation Fast and Furious lead to more than headline-grabbing Washington 'gotcha' games and cynical political point scoring," Holder concluded.
Holder has the support of Sen. Patrick Leahy, D-Vermont, chairman of the committee, who opened the hearing by reciting successes by the Justice Department under Holder's leadership.
On Monday, Leahy tried to lower the heat on Holder by raising it on the Bush administration for Operation Wide Receiver. In a letter to the Justice Department inspector general, the senator asked whether the current investigation also will cover the Bush-era supervision of the ATF in 2006 when, he said, "hundreds of weapons apparently moved beyond the custody and control of the ATF and possibly into Mexico and Arizona."
Leahy said that then-Attorney General Michael Mukasey may have been briefed in the case in 2007.
Holder faces what is likely to be a much more hostile environment next month when he testifies before a House panel, where Republican Darrell Issa, R-California, will wield the gavel and control the questioning.
CNN's Tom Cohen contributed to this report.