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 administration of President George W. Bush. 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 U.S. 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 pushed through two measures holding Attorney General Eric H. Holder Jr. in civil and criminal contempt for his failure to turn over additional Fast and Furious documents.

Though the measures were largely rhetorical, as Holder's Justice Department shows no signs of pursuing the matter, they marked the first time that either the House or the Senate had held an attorney general in contempt.

"The inspector general's report confirms findings by Congress' investigation of a near-total disregard for public safety," Rep. Darrell E. Issa, R-Calif., said Wednesday. "It's time for President Obama to step in and provide accountability for officials at both the Department of Justice and ATF who failed to do their jobs."

The chairman of the House Oversight & Government Reform Committee, Issa has largely led congressional Republicans in their pursuit of the Fast and Furious case. In July, after multiple hearings, Issa and Sen. Charles E. Grassley of Iowa, ranking Republican on the Senate Judiciary Committee, issued their own 211-page investigative report.

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 E. 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 Bush administration efforts. Investigators worked with licensed firearms dealers in the Southwest to track the sale of AK-47-style assault rifles and other guns to suspected "straw" purchasers, and then sought to track the firearms as they moved through criminal networks.

Over the course of the investigation, agents recorded the purchases of about 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 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 A. Terry.

ATF whistle-blowers then brought public and congressional attention to a possible tie between the firearms-tracking probe and Terry's death.

The operation was undertaken "without adequate regard for the risk it posed to public safety in the United States and Mexico," Justice Department investigators concluded.