A former high-ranking FBI counterintelligence official has pleaded guilty to conspiring to violate sanctions on Russia by working (after he retired) for an oligarch he once investigated.

Appearing before a federal judge in New York City, Charles McGonigal said he was “deeply remorseful” for work he did in 2021 for the billionaire industrialist Oleg Deripaska, AP reports.

McGonigal told the judge he accepted over $US17,000 ($26,000) to help Deripaska collect derogatory information about another Russian oligarch who was a business competitor.

Deripaska has been under US sanctions since 2018, for reasons related to Russia’s occupation of Crimea.

Assistant US Attorney Rebecca Dell also accused McGonigal of trying to help Deripaska to get off the sanctions list, and claimed he was in negotiations along with co-conspirators to receive a fee of up to $US3 million ($4.65 million) to hunt for electronic files revealing hidden assets belonging to the oligarch’s business rival.

McGonigal pleaded guilty to a single count of conspiring to launder money and violate the International Emergency Economic Powers Act.

He could face up to five years in prison, and is likely to be sentenced in December.