Washington (CNN) -- A top official in the Obama administration's Justice Department told a Senate panel Tuesday that he made a mistake in failing to warn Attorney General Eric Holder and other Justice executives about the controversial ATF tactic of allowing illegally purchased guns to "walk" across the border, into the hands of Mexico drug cartels.
At a Senate Judiciary subcommittee hearing, Assistant Attorney General Lanny Breuer was grilled by Sen. Charles Grassley, R-Iowa. Breuer acknowledged that in hindsight, he would have done things differently.
Breuer's apologetic tone followed months of top Justice Department officials staunchly defending what they said was their minimal role in flawed operations by the Bureau of Alcohol, Tobacco, Firearms and Explosives and denials of high-level involvement.
Breuer said he learned of the tactic of allowing illegally purchased guns to leave gun shops in April 2010 regarding a previous operation known as Wide Receiver.
"I wish that at that time that I had said clearly to the deputy attorney general and the attorney general that in this case, Wide Receiver, we had determined in 2006 and 2007 guns had 'walked.' I did not do that, and I regret not doing that," Breuer admitted.
Breuer had expressed his regret over his action in a statement late Monday.
Operation Wide Receiver began in 2006 during the Bush administration. The controversial Operation Fast and Furious began in 2009 during the Obama administration and allowed illegally purchased firearms to "walk" from gun stores in Arizona across the border to drug cartels with the intent of monitoring the flow of weapons, but hundreds were lost or unaccounted for.
The tactic led to a storm of outrage inside and outside the ATF when two of the missing weapons were found at the site where Border Patrol agent Brian Terry was killed in December.
Breuer also admitted to Grassley he had failed to recognize that the same tactics used four years earlier were used again in Fast and Furious.
"I did not draw a connection between the unacceptable tactics used by the ATF years earlier in Operation Wide Receiver and those in Operation Fast and Furious and therefore did not alert others within Department leadership of any similarities between the two. That was a mistake, and I regret not having done so," Breuer said.
As head of the criminal division, Breuer is one of Holder's most important assistants.
Breuer's statement of regret came as the Justice Department sent 650 pages of documents to congressional investigators. The documents had been subpoenaed October 11 by Republican lawmakers angered by what they claim was stonewalling by the Justice officials.
Grassley said at the hearing that the Justice Department officially assured him in February that allegations of gun walking in Fast and Furious were not true.
"However, as Mr. Breuer's admissions in yesterday's statement made clear, the Department's claim was not true," Grassley said.
"With every document that comes out, the Justice Department loses credibility and the faith of the American people," Grassley concluded.