Attorney General Eric Holder said Tuesday that he has not spoken with the family of Border Patrol agent Brian Terry, who was killed in December by a group of assailants using at least two weapons tied to "Operation Fast and Furious," but he "certainly regrets" what happened.

Not offering an outright apology when prompted by a Republican senator to offer one, Holder, who was appearing Tuesday before Congress for the first time since his controversial testimony in May over the gun-running program, said he "can only imagine" the Terry family's pain. But, he said, it's "not fair to assume" that mistakes made during the botched operation "directly led" to Terry's death.

Holder was on the hot seat at a Senate Judiciary Committee hearing in which he insisted that testimony he gave at his last appearance was accurate, but acknowledged that initial statements by the Justice Department after the issue became public were not.

In February, the Justice Department sent a letter to lawmakers saying every effort is made to "interdict weapons that have been purchased illegally and prevent their transportation to Mexico."

But details of "Fast and Furious" and similar investigations under the Bush administration have since come to light that reveal otherwise. Sen. Charles Grassley, R-Iowa, the ranking member on the panel, said "mounting evidence" in the months since the letter has "put the lie" to Justice Department claims.

The information in the letter was "inaccurate," Holder said, adding, "I regret that."

Holder said the Justice Department in Washington was "relying" on information provided by "people we thought in the best position to know," namely officials with the U.S. Attorney's Office and Bureau of Alcohol, Tobacco, Firearms and Explosives in Arizona.

Tuesday's hearing was intended to be a more general "oversight" hearing, but Holder was pummeled with questions over tactics used by ATF investigators in Arizona to target major gunrunners and his department's role in the matter.

Launched in late 2009, "Fast and Furious" planned to follow gun purchasers in hopes that suspects would lead them to the heads of Mexican cartels. But high-powered weapons tied to the investigation ended up at crime scenes in Mexico and the United States, including Terry's murder.

"Any instance of so-called 'gun walking' is unacceptable," Holder said in his opening remarks. He added that "Fast and Furious" was "flawed" and "should never have happened."

"Unfortunately, we will feel its effects for years to come as guns that were lost during this operation continue to show up at crimes scenes both here and in Mexico," he said. "It must never happen again."

Thirty-four House members and the National Rifle Association have called for Holder to resign.

Last month, Grassley distributed five memos addressed to Holder in July and August 2010, citing the gunrunning investigation by name. Nearly a year after those heavily redacted memos were sent, the attorney general in May told lawmakers under oath he "probably heard about 'Fast and Furious' for the first time over the last few weeks."

In his opening remarks, Grassley noted that in January -- the month after Terry was killed -- he handed Holder two letters mentioning "numerous allegations" from whistleblowers "that the ATF sanctioned the sale of hundreds of assault weapons to suspected straw purchasers" and "two of the weapons were then allegedly used in a firefight … killing CBP Agent Brian Terry."

The letters, addressed to then-ATF head Ken Melson, did not cite Fast and Furious by name.

While Holder asked for a retreat from "headline-grabbing Washington 'gotcha' games and cynical political point scoring," he said he did not mislead Congress in May over when he first heard of "Fast and Furious."

Holder said he first learned about tactics and the phrase "Operation Fast and Furious" beginning this year when it became a matter of public controversy.

"In my testimony I did say a few weeks, I probably could have said a couple of months," Holder said. "The focus of which day on which month is a bit of distraction (and) does nothing to address what concerns us most" -- the flow of guns, Holder said.

Later at the same hearing, in response to a follow-up question, Holder said he was probably aware of the operation at least six weeks earlier, by the time President Obama talked about it with a Spanish-language media outlet in late March.

Still, after the release of the memos last month, Republicans said Holder had "failed to give Congress and the American people an honest account of what he and others knew about gun-walking and 'Operation Fast and Furious.'"

Responding to such criticism from Republicans, Holder said Tuesday he doesn't believe his May testimony was inaccurate "based on what happened."

He noted receiving Grassley's letters in January and said he asked his staff to look into the matter before hearing more about it in the press. In late February, Holder asked the Justice Department's Inspector General to investigate "Fast and Furious," and in March he directed the department to issue guidance that all prosecutors refrain from "flawed tactics" like those of "Fast and Furious," Holder said.

"Clearly by the time I testified in May, I had known about 'Fast and Furious' for several weeks, (even) a couple of months," Holder said.

During his testimony Tuesday, Holder said Congress is not doing enough to support the agency responsible for the botched operation.

"The mistakes of 'Operation Fast and Furious,' serious though they were, should not deter or distract us from our critical mission to disrupt the dangerous flow of firearms along our Southwest border," Holder told the Senate Judiciary Committee.

Agents from ATF have told Congress their agency "suffers from a lack of effective enforcement tools," and a "critical first step should be for congressional leaders to work with us to provide ATF with the resources and statutory tools it needs to be effective," Holder said.

"Unfortunately, earlier this year the House of Representatives actually voted to keep law enforcement in the dark when individuals purchase multiple semi-automatic rifles and shotguns in Southwest border gun shops," Holder added.

Over the summer, the House passed an amendment blocking federal funds from being used by the ATF to require thousands of gun stores in California, Arizona, New Mexico and Texas to report multiple sales of certain weapons.

In his opening remarks, the ranking Republican on the Senate Judiciary Committee, Sen. Charles Grassley, R-Iowa, said some "seek to use" the tragedy of Terry's death "to call for new gun control."

"The bottom line is that it doesn'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 said, according to prepared remarks released by his office. "While trafficking in firearms is a real problem in Mexico, blaming our Second Amendment freedoms in the U.S. isn't accurate and won't fix anything."

On Monday, Senate Judiciary Committee Chairman Pat Leahy, D-Vt., sent a letter Monday to the Justice Department's inspector general, wanting to know if the inspector general's office would be looking at Bush-era operations as part of its inquiry into "Fast and Furious."

Recently disclosed documents show the ATF in Arizona lost track of hundreds of guns between 2006 and 2007 during investigations that involved so-called "gun-walking."

Last week, the head of the Justice Department's Criminal Division, Assistant Attorney General Lanny Breuer, issued a statement saying he learned of one of those investigations, "Operation Wide Receiver," in April 2010, and he now regrets not saying anything to Holder or others within the department at the time.

On Tuesday, Grassley said it's "bad enough that the head of the Criminal Division admits" the department's earlier claims it always tries to interdict weapons "was false."

"It gets worse, though," Grassley said. "(Breuer) remained silent for nine months as the public controversy over gunwalking grew. He was aware that Congress had been misled and yet made no effort to correct the department's official denial.

In addition to the inspector general's investigation into "Fast and Furious" and a congressional probe into the matter, the chairman of the House Judiciary Committee, Rep. Lamar Smith, R-Texas, is calling for a special counsel to investigate. Holder will be appearing before Smith's committee in December for a hearing focused specifically on "Fast and Furious."