The troubles that led to the ouster of Andrew McCabe as FBI deputy director began the same day that his boss, James B. Comey, was fired, when FBI investigators exploring media leaks approached McCabe for a conversation, people familiar with the matter said.

The investigators, who reported to McCabe, wanted to know about his dealings with the press, the people said. The Justice Department inspector general now thinks that McCabe was not completely truthful with them, the people added.

The episode figures prominently in a forthcoming inspector general report expected to allege that McCabe authorized a disclosure of information to the Wall Street Journal and then misled investigators about it. Though the Justice Department inspector general thinks that McCabe was not candid on multiple occasions — both under oath and not — the May 9 conversation is thought to be one of the first examples.

McCabe — who was fired from the FBI last week 26 hours before he could retire with full benefits — acknowledges that he authorized two FBI officials to talk to a reporter, though he denies that doing so was inappropriate and claims he “answered questions truthfully and as accurately as I could amidst the chaos that surrounded me.” The inspector general has not yet released the report detailing the allegations against McCabe, though it has been described by people familiar with its contents who, like others, spoke on the condition of anonymity.

McCabe has alleged that his firing is a political attempt to undermine the FBI and special counsel Robert S. Mueller III, who is probing whether the Trump’s campaign coordinated with Russia to influence the 2016 election. President Trump has made his distaste for McCabe well known and seized on his termination to take aim at the Russia probe, which he has called a witch hunt.

Lacking candor — the offense for which McCabe was fired — is considered a grave transgression in federal law enforcement. FBI agents deemed untrustworthy might have trouble testifying in a criminal case, so the bureau considers them almost unemployable.

Those who mislead the FBI also can face criminal charges. Several former Trump advisers, including former national security adviser Michael Flynn, have pleaded guilty to lying to federal law enforcement officials.

It is difficult to ascertain from what is known publicly so far whether McCabe’s alleged misdeeds could be criminal in nature. That the inspector general’s report is not yet released has fueled speculation inside the FBI and Justice Department, and on Capitol Hill, that prosecutors might be weighing a criminal case. If so, it is unlikely that the report would be made public until a decision has been made.

The inspector general also is examining broader allegations of misconduct surrounding the FBI’s handling of the investigation into Hillary Clinton’s use of a private email server while she was secretary of state, and it is possible that officials are mulling whether to release all of their findings at the same time.

On Monday, Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) pledged to hold a hearing on the report’s substance and the grounds for McCabe’s firing once the report is released. Grassley did not say whom he would ask to testify, though others on the committee have said that McCabe, FBI Director Christopher A. Wray and Attorney General Jeff Sessions should speak to the panel.

A spokesman for the inspector general has repeatedly declined to comment.

McCabe authorized two FBI officials, the FBI’s top spokesman and FBI lawyer Lisa Page, to talk to a Wall Street Journal reporter in October 2016, for a story the reporter was preparing on the Clinton email case and a separate investigation of the Clinton Foundation. The Wall Street Journal report was written by Devlin Barrett, who now is a reporter for The Washington Post. McCabe has said publicly that he felt he was being “accused of closing down investigations under political pressure,” and he wanted to push back.

Background conversations with reporters are commonplace in Washington under such circumstances, and McCabe, as the FBI’s No. 2 official, had the authority to approve such talks. The Justice Department inspector general, though, came to view the matter as an “unauthorized disclosure” — in part because the discussion was of an ongoing criminal investigation. Worse for McCabe, the inspector general concluded that the deputy director had not been honest when asked about the discussion.

McCabe has said that “when I thought my answers were misunderstood, I contacted investigators to correct them.”

McCabe’s credibility is important not only because he was a supervisor in the Russia investigation but because he could be a key witness — particularly as Mueller probes whether the president obstructed justice in his dealings with top law enforcement officials. Comey, whom Trump fired as FBI director, has alleged that Trump asked him for loyalty and whether he could let go of the investigation into Flynn. McCabe has said he could corroborate Comey’s accounts.

People familiar with the matter said that McCabe kept contemporaneous memos documenting his interactions with the president, who criticized McCabe’s wife and asked McCabe for whom he voted in the 2016 election. Jill McCabe, a Democrat, ran for a seat in the Virginia Senate and took donations from a Clinton ally — which has been a particular source of frustration for the president.