Months after it became a scandal, the Justice Department’s inspector general found a “pattern of serious failures” in the management of a fatal gun-running caper known as “Fast and Furious.” It should signal major changes at Justice.

So far, two top Justice Department officials have stepped down. But the IG’s report, released Wednesday, absolved Attorney General Eric Holder of any knowledge of the operation until it became a public scandal in January 2011. But that ignorance in itself is a scandal.

The undercover operation conducted by the Justice Department Bureau of Alcohol, Firearms and Tobacco allowed thousands of powerful weapons to be transported to Mexican gangs in 2009 and 2010. Those weapons have been involved in many Mexican deaths and the murder of U.S. Border Patrol Agent Brian Terry.

The operation violated U.S. law and damaged U.S.-Mexican relations. Low-level officials in AFT tried to have it called off, but were rebuffed by superiors who dismissed concerns about public safety because they thought, wrongly, that they would catch major gun-runners.

Inspector General Michael Horowitz found that the acting head of ATF, Kenneth Melson, was “deficient” in overseeing the operation, “ineffective” in closing it down, and provided insufficient information on the operation to his Justice Department overseers. Mr. Melson retired from Justice after the IG report was made public.

The IG found that Deputy Assistant Attorney General Jason Weinstein, an aide to the head of the Criminal Division, had access to information that should have informed him of the illegal activity in Operation Fast and Furious but failed to act. Mr. Weinstein was forced to resign this week, and claimed he had been made a scapegoat.

Indeed, administrative failures did not stop with Mr. Melson and Mr. Weinstein. The IG wrote, “We concluded that the Attorney General’s Deputy Chief of Staff, the Acting Deputy Attorney General, and the leadership of the Criminal Division” all failed to alert their boss, Eric Holder, to information they had concerning evidence that the ATF was illegally allowing guns to be transferred to Mexico.

Their collective failure came to an ugly head in a letter sent out on Feb. 4, 2011, under the signature of the Assistant Attorney General for Legislative Affairs.

It was the department’s formal reply to letters sent by Sen. Charles Grassley, R-Iowa, to acting ATF Director Melson regarding allegations from whistleblowers that guns the ATF allowed to be transferred into Mexico were used to kill Border Patrol agent Brian Terry.

The letter from Justice to Grassley said the allegations were “false.” But the IG’s report shows that senior Justice officials who reviewed the letter before it was sent had reason to believe otherwise and that the Grassley letters actually pointed to evidence supporting the allegations.

Sen. Grassley and House Republicans saw Justice officials trying to pull a fast one, and they have been furiously trying to get at the truth ever since.

Mr. Holder claims the IG’s report vindicates his assertions that the operation did not originate with him, and that he wasn’t involved in any coverup of Fast and Furious. But the question remains how such a hazardous operation could have escaped the attorney general’s notice. Or how he could have allowed such serious allegations concerning the murder of a federal agent to be cavalierly dismissed for so long.

As Mr. Horowitz testified this week, “There needs to be supervision; there needs to be oversight.” His report should give the department some direction toward that end.