Special counsel Robert Mueller may have a potential conflict of interest in the Russia investigation he is leading at the Department of Justice, it's alleged in a new opinion piece.

John Solomon of The Hill wrote Monday evening that a Russian being investigated as part of the probe has ties to Mueller back when the latter served as director of the FBI.

The FBI asked Russian oligarch Oleg Deripaska for help in trying to find and rescue retired FBI agent Robert Levinson, who was taken hostage in Iran in 2007 while he was doing work for the CIA. Deripaska ended up spending $25 million on the search, an operation that was nearly successful in 2010 before the State Department was unhappy with Iran's terms of Levinson's release. A deal that had been brokered was called off.

One of the FBI agents that helped lobby Deripaska for the task was Andrew McCabe, who eventually rose to deputy director but was fired earlier this year days before he was slated to retire.

The FBI's contract with Deripaska regarding the Levinson operation ended in 2011. Years later, Deripaska is one of the Russians under the microscope as part of the DOJ's investigation into whether the Trump campaign colluded with Russia.

It's already been reported that Deripaska had a business relationship with former Trump campaign manager Paul Manafort, who is a central player in Mueller's probe. Solomon reported Monday that the FBI contacted Deripaska soon after it learned of the infamous dossier written about then-candidate Donald Trump, a document that made scandalous but unverified claims about Trump and his alleged relationship with the Russians.

FBI agents even visited him in New York in 2016 when he was in the U.S. as part of a Russian delegation to the United Nations to ask him about Trump-Russia collusion, which the oligarch shrugged off as nonsense.

It is Manafort's past dealings with Deripaska that are now being scrutinized at the DOJ, according to Solomon. He was even sanctioned last month over election meddling allegations, although the FBI appeared to trust him enough over the years to use him as a key source and allow him to travel to the U.S. nearly 10 times.