New York, New York - A former top FBI official was sentenced to more than four years in prison for working for a sanctioned Russian oligarch, US authorities said on Thursday.

Charles McGonigal (55) was sentenced to 50 months in prison and fined $40,000 for conspiring to violate sanctions laws and commit money laundering related to his work with Oleg Deripaska.

He had pleaded guilty in August after being charged in January.

McGonigal, a former special agent in the New York Counterintelligence Division, had helped investigate Deripaska and other oligarchs, and supervised investigations into sanctions violations during his time at the FBI, the Justice Department said in a statement.

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"Yet at the same time, he began building a relationship with an agent of Deripaska, in the hopes of doing business with Deripaska after he retired from the FBI," it said.

Part of the work that McGonigal carried out for Deripaska included investigating a rival Russian oligarch, "in return for concealed payments."

"While negotiating and performing services for Deripaska, McGonigal and the agent attempted to conceal Deripaska's involvement," including by using shell companies and forged signatures, the Justice Department said.

"Charles McGonigal violated the trust his country placed in him by using his high-level position at the FBI to prepare for his future in business," said US Attorney Damian Williams.

"Once he left public service, he jeopardized our national security," he added, calling Deripaska "Vladimir Putin's agent."

The United States slapped sanctions on Deripaska in 2018 over his close ties to Russia's government and its seizure four years before of the Crimea region from Ukraine.

McGonigal had retired that same year.