WASHINGTON (AP) — Attorney General Eric Holder on Tuesday acknowledged serious mistakes in an arms-trafficking probe that allowed AK-47s and other weapons to leak into the black market, but he insisted the Justice Department was taking steps to ensure that never happens again.Under pointed questioning by Republicans, Holder also expressed regret that the Justice Department had denied allegations of "gun-walking" in a letter to Iowa Sen. Charles Grassley sent earlier this year."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 told the Senate Judiciary Committee of the investigation, known as Operation Fast and Furious.Grassley, the panel's top Republican, said the operation represented an "utter failure" by federal law enforcement officials to enforce existing gun laws.The purchases of more than 2,000 weapons aroused the suspicion of Fast and Furious investigators, but the suspected straw buyers of those guns were allowed to walk out of Phoenix-area gun shops with AK-47s and other weapons, rather than being arrested.The goal was to track those weapons to gun-trafficking ring leaders, suspected to include Mexican drug lords, who had long eluded prosecution. But agents lost track of about 1,400 of the guns. As of Oct. 20, 276 guns in Fast and Furious have been recovered in Mexico and 389 recovered in the United States.The letter from the Justice Department to Grassley in February said federal agents make "every effort" to intercept weapons that have been purchased illegally and prevent their transportation to Mexico. Holder said the letter to Grassley was based on information the Justice Department received from the U.S. attorney's office in Phoenix and the Bureau of Alcohol, Tobacco, Firearms and Explosives in Washington."It's unconscionable that a federal agency would let such a misleading letter stand for more than nine months," Grassley said following the hearing.Pressed by Sen. John Cornyn, R-Texas, Holder also expressed regret to the family of a slain federal border agent whose death prompted ATF agents to come forward early this year and provide information to Grassley about Fast and Furious.The attorney general added, however, that it is not fair to assume that mistakes in Operation Fast and Furious led to the death of Customs and Border Protection agent Brian Terry. Two of the guns whose purchase was identified by Operation Fast and Furious investigators turned up at the scene of a shootout in Arizona that resulted in the death of Terry, a Customs and Border Protection agent.Holder said he first heard allegations of problems in Fast and Furious early this year, prompting Cornyn to ask whether it was Holder's responsibility to have known earlier."I have ultimate responsibility for that which happens in the department, but I cannot be expected to know the details for every operation that is ongoing in the Justice Department on a day-to-day basis," replied Holder.Holder has been criticized in a congressional investigation by the House Oversight and Government Reform Committee, chaired by Rep. Darrell Issa, R-Calif. Republicans have suggested Holder was informed of the problems as early as July 2010 when the operation's name turned up repeatedly in weekly departmental reports — a point raised Tuesday by Cornyn. Those reports provided updates on dozens of investigations, including Fast and Furious, but do not mention the gun-walking tactic.The Associated Press has reported that the investigation of Fast and Furious has turned up Justice Department documents which indicate the gun-walking tactic was used in two other investigations by ATF offices in Arizona during the Republican administration of George W. Bush and a briefing memo to Bush's Attorney General Michael Mukasey that briefly described use of the tactic. Mukasey has declined to comment on the memo.In August, Holder replaced three officials who played critical roles in the arms-trafficking probe — the acting director of ATF, the U.S. attorney in Arizona and a prosecutor who worked on the arms trafficking probe. The department's inspector general is still investigating the case, at Holder's request. The ATF's new director has revamped the management structure at the agency's headquarters.Democrats raised the issue of two Bush-era probes that involved gun-walking. Democratic Sen. Chuck Schumer said that Republicans are selectively focusing on the Obama administration through Fast and Furious and that if the Republican-controlled House Oversight and Government Reform Committee won't look into the earlier probes, "we should."Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., asked the IG whether its probe of Fast and Furious was being expanded to cover the older probes. One investigation begun in 2006, Operation Wide Receiver, involved 350 weapons, many of which fell into the hands of arms traffickers. In the second Bush-era probe, a briefing paper prepared for then-Attorney General Mukasey outlined failed attempts by federal agents to track illicitly purchased guns across the border into Mexico in a 2007 probe.Cornyn said that unlike Fast and Furious, Operation Wide Receiver involved coordination with Mexican law enforcement officials. But in the 2007 probe, email traffic among ATF officials said the investigation failed because Mexican law enforcement fell down on the job of tracking the weapons after they were told what vehicle was bringing them across the border. That memo urged Mukasey to press his Mexican counterpart to provide corruption-resistant agents for additional operations that would use the gun-walking tactic.The Justice Department inspector general's office said in a semiannual report that it was reviewing Operation Fast and Furious "and other investigations with similar objectives, methods and strategies." A spokesman for the IG's office, Jay Lerner, declined to comment on whether the 2006 and 2007 probes are part of the investigation.