The former head of counterintelligence for the F.B.I. in New York pleaded guilty in federal court in Manhattan on Tuesday to a single reduced charge of conspiring to violate U.S. sanctions and laundering payments from a prominent Russian oligarch.

The plea by the former agent, Charles F. McGonigal, represented a remarkable turn for a man who once occupied one of the most sensitive and trusted positions in the American intelligence community, placing him among the highest-ranking F.B.I. officials ever to be convicted of a crime.

Appearing before Judge Jennifer H. Rearden of Federal District Court on Tuesday, an emotional Mr. McGonigal stood up and said that he had broken the law after his retirement in 2018 from the bureau, where he had been an expert in Russian counterintelligence, by aiding an effort by Oleg V. Deripaska, a Russian billionaire under U.S. sanctions, to investigate a rival.

[Irony certainly can be quite ironic. McGonigal could get up to five years in prison under this plea deal, and he has another indictment still pending for taking money while still in the FBI from Eastern European sources. — Ed]