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 does n'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 .
DoJ officials took months to correct `` misleading '' info , Sen. Charles Grassley says . `` We are losing the battle to stop the flow of illegal guns to Mexico , '' Holder says . Attorney general says `` gun-walking '' tactics `` must never happen again '' Republican critic says existing gun control laws were n't enforced .