Attorney General Eric Holder vigorously denied a "cover-up" by the Justice Department over "Operation Fast and Furious," telling a House panel investigating the botched gun-running program that he has nothing to hide and suggesting the probe is a "political" effort to embarrass the administration.

"There's no attempt at any kind of cover-up," Holder told lawmakers well into a hearing about whether he had been forthright in responding to requests of the House Oversight and Government Relations Committee led by Chairman Darrell Issa, R-Calif.

"We're not going to be hiding behind any kind of privileges or anything," he said.

The hearing came after Issa and Sen. Charles Grassley, R-Iowa, his Senate partner in the probe, asserted that top Justice officials are covering up events surrounding the flawed gun-smuggling probe.

Issa made the accusation in a letter threatening to seek a contempt of Congress ruling against Holder for failing to turn over congressionally subpoenaed documents that were created after problems with Fast and Furious came to light.

Republicans also released a report in the hours ahead of the hearing claiming that Justice Department officials "had much greater knowledge of, and involvement in, Fast and Furious than it has previously acknowledged."

Asked whether his assistants, Deputy Attorney General Gary Grindler or Assistant Attorney Lanny Breuer, head of the department's Criminal Division, ever authorized gunwalking or the tactics employed in Fast and Furious, Holder responded not to his knowledge.

"Not only did I not authorize those tactics, when I found out about them I told the field and everybody in the United States Department of Justice that those tactics had to stop. That they were not acceptable and that gunwalking was to stop. That was what my reaction [was] to my finding out about the use of that technique," he added.

He added that he doesn't think that the situation warranted the kind of response Republicans were giving after his department provided thousands of documents, and planned to deliver more.

Holder also rejected arguments that his handling of the case had lost him any support for the effort he was putting forth as attorney general.

"I don't think the American people have lost trust in me. ... This has become political, I get that," he said.

But Holder also said no one has been punished "yet" in the case, despite the fact that lost guns from the operation ended up at the crime scene where U.S. Border Patrol Agent Brian Terry was murdered in December 2010.

Terry's family has informed the U.S. government that it has six months to respond to its inquiry into Terry's death or face a $25 million lawsuit.

In the botched operation, more than 1,400 weapons sold to low-level straw purchasers believed to be supplying Mexican drug gangs and other criminals were lost during tracking by Bureau of Alcohol, Tobacco, Firearms and Explosives agents. Another 700 firearms connected to suspects in the investigation have been recovered, some from crime scenes in Mexico and the U.S., including in Nogales, Ariz., where Terry was killed.

Holder said he didn't learn about Terry's murder until 24 hours after his death, and at the time did not hear that weapons tied to Fast and Furious were at the scene.

"I didn't know about Operation Fast and Furious until the beginning parts of 2011 after I received that letter from Senator Grassley, I guess at the end of January and then that was about Operation Gun Runner. I actually learned about the Fast and Furious operation in February of that year."

Holder told the committee, "I’m not sure exactly how I found out about the term, 'Fast and Furious.'" He testified repeatedly that he never authorized the controversial tactics employed in the operation.

"There is no attempt at any kind of cover-up," Holder said. "We have shared huge amounts of information" and will continue to do so, he said.

But Holder said under questioning that he has not disciplined anyone for his role in the controversial operation.

"No I have not as yet -- as yet," Holder said when questioned by Issa on the matter. "There have been personnel changes made at ATF. We obviously have a new U.S. attorney in Arizona. We have made personnel switches at ATF. People have been moved out of positions."

Holder's statements on the Justice Department's role in the operation did not sit well with Republican lawmakers on the committee, who accused the attorney general of intentionally withholding key documents in the case.

"The conclusion that I come to is there are some things in there that's being hidden that you don't want us to see," said Rep. Dan Burton, R-Ind. "We have every right under the Constitution to check on what you're doing... So for you to deny this committee anything like that is just dead wrong and I don't think you're going to find any way that you can do it."

Burton went on to say that 93,000 documents related to the operation are being withheld by the Justice Department even though they've been turned over internally to the department's inspector general, a political appointee, Burton said.

"And you're saying, well, the separation of powers prohibits you from (delivering them to Congress). That's baloney. That is just baloney," Burton said.

Rep. Jason Chaffetz, R-Utah, also questioned Holder's having not discussed the case with Secretary of State Hillary Clinton or Homeland Security Secretary Janet Napolitano.

"When people know that I'm going to be the subject of these kinds of hearings, you know six times and all that, nobody necessarily wants to get involved in these kinds of things or get dragged into it," Holder responded.

Issa told Holder the committee will do what is necessary to obtain the information, "If you do not find a legitimate basis to deny us the material we've asked for."

Holder said earlier during testimony that he would release additional materials "to the extent that I can."

In Holder's defense, Rep. Edolphus Towns, D-N.Y., claimed the committee has "not obtained one shred of evidence that would contradict your testimony."

"Not one witness, not one document, not one e-mail, and still some continue to suggest that you did personally authorize gunwalking and the tactics in Operation Fast and Furious."