U.S. Attorney General Eric Holder should have handed over thousands of documents that he repeatedly refused to show to lawmakers investigating the botched Fast and Furious gun-tracking operation, a Justice Department investigator said Thursday.

Justice Department Inspector General Michael Horowitz told the House Oversight and Government Reform Committee that he received 100,000 pages of documents related to the botched program. Republican lawmakers seeking the same information earlier this year were allowed to see fewer than 8,000 pages, and many of those had large sections blacked out. The standoff culminated in July with House Republicans voting to find Holder in contempt of Congress.

Republicans used Thursday's hearing to justify their contempt vote with testimony that Holder had deliberately withheld from Congress documents relevant to its investigation.

"Do you know of pages that you saw that Congress should, for good cause, be denied?" Chairman Darrell Issa, R-Calif., asked Horowitz during a three-hour hearing at which he was the only witness.

"Every document we asked for and reviewed and cited in this report we found to be relevant and important," Horowitz responded.

Horowitz noted that his report cites the Justice Department for "failing to respond accurately to a congressional inquiry" about Fast and Furious.

Horowitz's testimony all but concluded a months-long investigation by lawmakers into Fast and Furious, a Bureau of Alcohol, Tobacco, Firearms and Explosives operation in which agents allowed thousands of American guns to be sold to violent Mexican drug cartels. Officials had intended to trace the guns to the cartels, but lost track of many of them. Those guns were later connected to the murder of U.S. Border Patrol Agent Brian Terry as well as hundreds of Mexicans.

Horowitz testified a day after releasing a 471-page report that labeled Fast and Furious "a series of misguided strategies, tactics, errors in judgment, and management failures" at ATF and the Arizona District U.S. Attorney's Office.

While the report spread blame to a number of Justice Department officials -- prompting at least two resign Wednesday -- it essentially exonerated Holder by concluding that he was unaware of the program, though he should have employed greater oversight of the lower-level officials who devised it.

Republicans had been eager to see the withheld documents in part because some believed Holder knew more about the program than he admitted and was trying to cover up his involvement.

At Thursday's hearing, however, Democrats made a point of underscoring Horowitz's conclusion that Holder, in fact, did not know about Fast and Furious. Issa agreed, pointing out that the report also excludes from blame Holder's predecessor, Michael Mukasey, a Republican appointee of President George W. Bush whose tenure occurred during a prior gun-walking program.

"I don't think anyone should have assumed that they knew," Issa said of Mukasey and Holder. "I think the inspector general's report does cast blame for high-ranking people not asking more question. But I agree ... that neither attorney general was found to know it."

Horowitz told lawmakers his investigation is continuing. He is now "actively investigating" reports that Fast and Furious whistleblowers within the department have been unfairly punished for coming forward.

"You'll find the reports are coming in the not-too-distant future and we are taking them very seriously," Horowitz said.