WASHINGTON – The controversial “Fast and Furious” gun operation that let U.S. firearms flow into Mexico was “seriously flawed” and poorly overseen, Justice Department investigators concluded Wednesday in a long-awaited report that caused immediate political casualties.

One senior Justice Department official resigned, and another quickly retired in the wake of the 512-page report, which provided devastating details about the so-called gun-walking operation and its predecessor begun in the Bush administration. Up to a dozen or so other Justice Department officials face potential disciplinary action.

“Our review of Operation Fast and Furious and related matters revealed a series of misguided strategies, tactics, errors in judgment and management failures,” the Justice Department’s Office of the Inspector General concluded.

Fast and Furious – and the earlier Operation Wide Receiver, begun in 2006 – entailed investigators from Justice’s Bureau of Alcohol, Tobacco, Firearms and Explosives allowing guns to be sold in the United States to illegal buyers. The hope was that the weapons ultimately could be tracked to Mexican drug cartel leaders and other top-level gangsters.

Investigators subsequently found some of the guns at U.S. crime scenes, including one that involved the fatal shooting of a Border Patrol agent.

The problems, investigators found, “permeated” the ATF headquarters, as well as a Phoenix field division and the federal prosecutor’s office in Arizona. They extended, as well, to relations with Congress, as investigators noted that Justice Department officials had provided “misleading” statements to lawmakers.

In June, reflecting the partisan distrust, Republicans in the House of Representatives pushed through two measures holding Attorney General Eric Holder in civil and criminal contempt for his failure to turn over additional Fast and Furious documents.

The new inspector general’s report, though, adds considerable nonpartisan clarity to the investigations undertaken by Congress. The inspector general’s investigators acknowledged that Holder had taken some positive steps, and they didn’t find his fingerprints on the ill-conceived Fast and Furious operation itself.

“The key conclusions are consistent with what I, and other Justice Department officials, have said for many months now,” Holder said in a statement.

“The leadership of the department did not know about or authorize the use of the flawed strategy and tactics, and the department’s leadership did not attempt to cover up information or mislead Congress about it.”

Nonetheless, after the report was released, Holder announced the immediate retirement of Kenneth Melson, the former acting director at ATF, as well as the resignation of Deputy Assistant Attorney General Jason Weinstein, a longtime career prosecutor. Other potential disciplinary actions are pending.

The firearms trafficking investigation that became known as Fast and Furious began in 2009, an outgrowth of similar efforts under former President George W. Bush.

Investigators worked with licensed firearms dealers in the Southwest to track the sale of AK-47-style assault weapons and other guns to suspected “straw” purchasers, and then sought to track the firearms as they moved through criminal networks. Investigators hoped the guns would lead them to crime bosses instead of underlings.

Over the course of the investigation, agents recorded the purchases of some 2,000 weapons, with about $1.5 million changing hands. Because of the decision to let the guns get into the hands of bigger investigative targets, U.S. officials seized only about 100 weapons during the operation. The rest disappeared into the criminal underworld.

The operation ended in January 2011, a month after firearms that had been sold as part of it were found in a remote Arizona canyon, the scene of the fatal shooting of Border Patrol Agent Brian Terry.