Washington - A former senior FBI agent who once investigated Russian oligarch Oleg Deripaska has been arrested for receiving secret payments from the Russian billionaire in return for investigating a rival, the Justice Department announced Monday.

Charles F. McGonigal, who headed counterintelligence for the FBI's New York field office, and Sergey Shestakov, a former Soviet diplomat and an associate of Deripaska, were arrested Saturday on sanctions violations and money laundering charges.

According to court documents, McGonigal and Shestakov in 2021 investigated an unnamed rival Russian oligarch in return for concealed payments from Deripaska, violating U.S. sanctions imposed on Deripaska in 2018.

In an earlier scheme in 2019, McGonigal and Shestakov allegedly unsuccessfully tried to have the sanctions against Deripaska lifted, according to court documents.

Before retiring from the FBI in 2018, McGonigal, a veteran special agent, led and participated in investigations of Russian oligarchs, including Deripaska, according to the Justice Department.

Shestakov was a Soviet and Russian diplomat before becoming a U.S. citizen and a Russian interpreter for U.S. courts and government offices, according to the Justice Department.

According to court documents, the duo tried to conceal Deripaska's involvement in the investigation of his rival by using shell companies, not naming the businessman in communications and forging signatures.

In September, Deripaska, an aluminum magnate with suspected ties to Russian President Vladimir Putin, and three associates were indicted on charges of evading U.S. sanctions and obstruction of justice.

Deripaska remains at large.