A senior Justice Department official was sharply questioned by a Republican senator on Tuesday over his admission that he knew about a controversial anti-gun-trafficking tactic used during the Bush administration and did not alert his supervisors.

Assistant Attorney General Lanny A. Breuer told a Senate Judiciary subcommittee that he learned last year that the tactic of allowing guns to flow illegally onto U.S. streets and into Mexico was used by federal agents between 2006 and 2007. The tactic, known as “gun walking,’’ was also a key part of “Fast and Furious,’’ the Obama administration gun-trafficking operation that is under investigation by Republican lawmakers and the Justice Department’s inspector general.

In a statement Monday, Breuer said he regretted not telling other Justice officials about gun walking that occurred during “Operation Wide Receiver,” a gun-trafficking initiative in the Bush administration’s Justice Department. Calling the tactic “unacceptable and misguided,’’ Breuer wrote that he should have made the connection between Wide Receiver and Fast and Furious when controversy over Fast and Furious erupted this year. The operations came out of the same U.S. attorney’s office in Phoenix.

Under questioning from Sen. Charles E. Grassley (R-Iowa), Breuer repeatedly expressed regret and said he thought last year that having aides discuss his concerns with the Bureau of Alcohol, Tobacco, Firearms and Explosives — which ran both operations — was “the appropriate way of dealing with it.” He added that “I cannot be more clear: If I had known then what I know now, of course I would have told” Attorney General Eric H. Holder Jr.

Breuer said he had not known that ATF also allowed guns to “walk’’ — meaning they could pass into the black market or Mexico without ATF intervention — in Fast and Furious. But Grassley said the admission about Wide Receiver reinforced his suspicions that senior Obama Justice officials were involved.

“The American people . . . deserve answers from the Justice Department about why they claim they didn’t know gun walking was occurring in Operation Fast and Furious when the department’s fingerprints are all over it,’’ Grassley said.

The question of whether senior officials were aware of the details of Fast and Furious lies at the heart of the dispute over the Phoenix-based gun sting, which began in 2009 and allowed small-time straw purchasers to pass firearms to middlemen, who then trafficked the guns to Mexico. The fury over the tactics, which resulted in more than 2,000 illegally purchased firearms hitting the streets, has led to the reassignment of ATF’s former acting director and others, and the resignation of Arizona’s U.S. attorney.

Holder has come under fire from Republican lawmakers, who question whether he was truthful when he said at a May 3 congressional hearing that he “probably heard about Fast and Furious for the first time over the last few weeks.” Justice officials have said Holder was truthful and that he first learned about the operation and its questionable tactics earlier this year.

On Tuesday, Justice officials characterized Breuer’s admission as a sincere expression of regret and said he retains Holder’s full confidence. In his statement Monday, Breuer, who oversees the department’s Criminal Division, said he remains “more committed than ever” to its “mission to fight violence in the United States and Mexico.”

Breuer won support from Democrats, including Rep. Henry A. Waxman (Calif.), who said Tuesday that Breuer “has had a phenomenal record” in his tenure at Justice. And Sen. Dianne Feinstein (D-Calif.) said at Tuesday’s hearing that while “perhaps mistakes were made” in Fast and Furious, Congress should focus more on tightening the nation’s porous gun laws.

Breuer said he agreed. “It’s clear that we need more tools to get those people who are buying guns and transporting them to Mexico . . . we need to stop the flow,’’ he said.