Smooth and politic while navigating an upward trajectory through the F.B.I.’s bureaucracy, he was a different man with subordinates, flashing his temper at the smallest provocation, former associates say. An expert in Russian counterintelligence, he spoke publicly of international security threats. At the same time, prosecutors say, he was privately courting the oligarch, Oleg V. Deripaska, who figured prominently in the investigation by the special counsel, Robert S. Mueller III, into Russian meddling in the 2016 U.S. presidential election.

Now, Mr. McGonigal, 55, appears set to become one of the highest-ranking F.B.I. agents ever to be convicted of a crime. He is scheduled to appear in federal court in New York on Tuesday for a possible guilty plea in the case involving Mr. Deripaska, and is in talks to resolve an indictment brought by federal prosecutors in Washington. Until any deal is finalized, it could still fall apart, and Mr. McGonigal, who has so far pleaded not guilty, could go to trial.

The case has raised unsettling questions about the F.B.I.’s ability to detect corruption within its ranks. Prosecutors suggested that Mr. McGonigal traveled extensively while at the bureau, meeting with foreign officials and businesspeople who, on the surface, had nothing to do with his job. Agents are required to report such contacts and certain financial transactions and to take lie-detector tests, but the bureau relies heavily on the integrity of the people it has placed in positions of trust.

Over more than three years, the investigation has so far produced no evidence that Mr. McGonigal provided national security secrets to the Russians or to anyone else, according to American officials who requested anonymity to discuss ongoing cases. Although the officials said Mr. McGonigal appears to have been engaged in simple graft, his actions stunned many in the F.B.I., where a core tenet is drilled into every agent: “Never embarrass the bureau.”

The F.B.I.’s director, Christopher A. Wray, said the charges demonstrated “the F.B.I.’s willingness as an organization to shine a bright light on conduct that is totally unacceptable, including when it happens from one of our own people, and to hold those people accountable.”

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