The US inspector general assigned to get to the bottom of the Fast and Furious “gunwalking” fiasco on Thursday debunked the notion that Attorney General Eric Holder knew about the program – or that it was a White House conspiracy to push for tougher gun laws.

But he did tell Congress that he was amazed how such a large project could be run with the assistance of the Department of Justice without the attorney general knowing about it. “We struggled to understand how an operation of this size and importance, with this potential impact on the country, could not have been briefed up to the attorney general,” Michael Horowitz said.

Republicans suggested that the report – together with a new willingness by Mr. Holder to share documents – should go a long way toward resolving a potentially drawn-out legal affair over the House's decision to hold Holder in contempt on civil and criminal charges in June. But Horowitz's investigation continues, and he told the House Oversight and Reform Committee Tuesday that he remains “troubled” by what he found inside key parts of the Justice Department.

He said the main fault lay in “tactical and strategic” decisions made by agents on the ground, but he was also concerned by:

The process by which deputy assistant attorney generals review wiretap affidavits.

The quality and thoroughness of communication inside the agency.

Evidence of retaliation against whistleblowers who revealed the government’s role in a segment of the borderland gun trade.

A pattern of spinning news as evidenced by interagency “emails that were more concerned about what the public’s reaction would be after learning about gunwalking.”

Begun in October 2009, Operation Fast and Furious was an expansion of a similar program run during the Bush administration three years earlier. The idea was to target high-ranking cartel members by allowing known straw purchasers to cross the border into Mexico and deliver the weapons. It allowed over 2,000 mostly AK-47s, worth $1.5 million, to “walk” without interdiction into Mexico. Only about 100 guns were recovered before crossing the border.

The operation did lead to indictments against 40 alleged gun-runners and criminals, but the guns run through Fast and Furious have been found at dozens of Mexican murder scenes as well as near where Border Patrol Agent Brian Terry was gunned down by bandits in December 2010.

In February 2011, a letter signed off on – but apparently never actually read – by Justice Department Criminal Division chief Lanny Breuer claimed that the US had never let guns “walk” across the border. Mr. Breuer reports directly to Holder. Ten months later, Holder was forced to retract that letter after it became clear that US agents had done exactly that – on a huge scale.

On Thursday, Horowitz pointed to concerns throughout the Justice Department, especially at the US attorney's office in Phoenix and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).

The actions of at least 14 US officials – from line agents to deputy assistant attorney generals – “reflected a significant lack of oversight and urgency and a disregard for safety of individuals in the United States and Mexico, [and highlighted] misguided strategies, tactics, errors in judgement and management failures that permeated ATF headquarters and headquarters of the Department of Justice,” Horowitz said. “There were many points in this case at all levels where information flow not only wasn’t what it should have been, but in some instances it was inaccurate even when information was flowing.”

In his report, Horowitz said that Assistant Attorney General Jason Weinstein reviewed a cover memo for one wiretap application that “clearly suggests” ATF agents had let a straw buyer continue to illegally traffic guns to cartels. Mr. Weinstein resigned Wednesday. Through his lawyer, Weinstein has said he had been assured by ATF staff that the guns were being interdicted.

Former ATF acting chief Ken Melson, who was also criticized in the report, retired from government service Wednesday. The 12 others mentioned will face employment reviews, according to Holder.

Rep. Elijah Cummings, the top Democratic member on the oversight committee, agreed that the report raised troubling questions. “You’ve painted the picture quite accurately for us, and nobody up here likes the picture that we see, and I don’t think Eric Holder likes this picture,” Congressman Cummings told Horowitz.

While maintaining he didn’t know about the program until after Mr. Terry’s death, Holder has taken responsibility for Fast and Furious, vowed it won’t happen again, and has apologized to the family of Terry. According to Horowitz, Holder has begun to address specific reforms suggested by the report – including the handling of wiretap affidavits – and Justice has to report back in 90 days on how those reforms are proceeding.

For his part, Rep. Darrell Issa (R) of California, one of Holder’s most consistent critics and chairman of the committee, agreed that evidence suggests Holder was not responsible for the program itself. What was at issue in the June contempt votes, other Republicans noted, was Holder's refusal to release documents Congress wanted. President Obama also invoked executive privilege to protect some of the documents, although Horowitz was able to access all 100,000 documents he requested.

Horowitz told the committee that his investigation continues, particularly into allegations of retaliation against whistleblowers. He’s also working to have courts unseal the specific Fast and Furious wiretap applications that he reviewed, and which, he says, raised “red flags” that Justice Department attorneys should have picked up on.

Horowitz also suggested another potential line of questioning for Congress. He was rebuffed by the lawyers of Kevin O’Reilly, a former White House national security staffer, in his efforts to discuss conversations Mr. O'Reilly had with a top ATF official in Phoenix that reference Fast and Furious. Horowitz could not force Mr. O’Reilly to talk because he didn't have subpoena purview outside the Justice Department.