Sen. Chuck Grassley (Iowa), the top Republican on the Senate Judiciary Committee, grilled Attorney General Eric Holder today regarding a botched weapons investigation known as “Fast and Furious” that allowed assault weapons to make their ways to Mexican drug cartels.

Holder provided a clear admission that law enforcement officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives intentionally stopped surveillance on guns they suspected were en route to criminal networks.

“Any instance of so-called gunwalking was unacceptable. This tactic was unfortunately used as part of Fast and Furious,” Holder told senators at a Judiciary Committee hearing. “This should never have happened.”

But Holder also took a strident tone against what he called “inaccurate and frankly irresponsible accusations” about the role of top Justice Department officials in approving the tactics used in the operation.

He also sought to pivot the revelations into a push for more stringent gun laws, noting that ATF whistle-blowers who testified at a House hearing in June “explained that the agency’s ability to stem the flow of guns from the United States into Mexico suffers from a lack of effective enforcement tools.”

Grassley pressed Holder about whether Justice Department officials had informed him when investigators tied Fast and Furious weapons to the scene of Border Patrol Agent Brian Terry’s murder.

Holder said they had not.

Grassley also asked about the status of Assistant Attorney General Lanny Breuer.

Breuer last week said he knew that federal officials allowed weapons to fall into the possession of Mexican drug cartels as early as April 2010, 10 months before the department denied to Congressional investigators that the investigative strategy was used.

On Oct. 31, Breuer released a statement expressing “regret” for not alerting other top officials about the tactic when it “became public.”

Holder said Breuer had not offered his resignation over the matter and he did not expect Breuer would be offering it.

Holder conceded that a Feb. 4 letter broadly denying the Justice Department had allowed guns to “walk” was inaccurate. He said it would be a “reason for concern” if Breuer had reviewed the letter before it was sent to Grassley.

Grassley also asked Holder about an instance in which an unknown Justice Department official leaked damaging information about an ATF whistle-blower that Grassley said appeared to be retaliatory.

The Iowa Republican said Holder indicated in a private phone conversation with him that an investigation had revealed who leaked the information and that the person in question had been dealt with.

Holder paused, saying, “it almost pains me” to respond to the question, since it involved what he thought was a private phone conversation between the two in response to a handwritten note Grassley had sent Holder.

“In a different time in Washington, I’m not sure … that would have been shared with everyone,” Holder said, ultimately answering the question by saying the leaks were “under investigation.”