New concerns have been raised regarding a possible conflict of interest in Special Counsel Robert Mueller’s Russia collusion probe, involving a Russian oligarch, the FBI, and its failed attempt to get him to assist in the investigation.

Russian oligarch Oleg Deripaska is a name that pops up from time to time in the collusion narrative, though it has been conspicuously absent from Mueller’s indictments. Now, a new report published at The Hill by investigative reporter John Solomon could help explain why: Deripaska has a history of working with Mueller’s FBI.

According to Solomon’s sources, the Bureau in 2009 asked the oligarch to spend millions of his own dollars to fund an FBI-supervised operation to rescue retired FBI agent Robert Levinson, who was captured in Iran while working for the CIA in 2007. The sources include more than a dozen participants inside and outside the FBI, the Levinson family, a retired agent who supervised the case, Deripaska’s lawyer, and Deripaska himself.

“We knew he was paying for his team helping us, and that probably ran into the millions,” a U.S. official involved in the operation confirmed:

One agent who helped court Deripaska was Andrew McCabe, the recently fired FBI deputy director who played a seminal role starting the Trump-Russia case, multiple sources confirmed. Deripaska’s lawyer said the Russian ultimately spent $25 million assembling a private search and rescue team that worked with Iranian contacts under the FBI’s watchful eye. Photos and videos indicating Levinson was alive were uncovered.

FBI officials told Solomon that the mission was hampered by Hillary Clinton’s State Department. Deripaska had his visa revoked by the State Department in 2006 amid concerns about his links to organized crime, which he has denied:

“We tried to turn over every stone we could to rescue Bob, but every time we started to get close, the State Department seemed to always get in the way,” said Robyn Gritz, the retired agent who supervised the Levinson case in 2009, when Deripaska first cooperated, but who left for another position in 2010 before the Iranian offer arrived. “I kept Director Mueller and Deputy Director [John] Pistole informed of the various efforts and operations, and they offered to intervene with State, if necessary.” FBI officials ended the operation in 2011, concerned that Deripaska’s Iranian contacts couldn’t deliver with all the U.S. infighting. Levinson was never found; his whereabouts remain a mystery, 11 years after he disappeared.

Some aspects of the story were chronicled in a 2016 book by reporter Barry Meier, according to Solomon.

Harvard law professor Alan Dershowitz told Solomon he believes Mueller has “a conflict of interest” because of the FBI’s past dealings with the Russian.

“The real question becomes whether it was proper to leave [Deripaska] out of the Manafort indictment, and whether that omission was to avoid the kind of transparency that is really required by the law,” Dershowitz said.

Deripaska’s ties to Mueller could prove to be embarrassing to Mueller, George Washington University constitutional law professor Jonathan Turley told The Hill. “If the operation with Deripaska contravened federal law, this figure could be viewed as a potential embarrassment for Mueller. The question is whether he could implicate Mueller in an impropriety,” he said.

“There’s questions as to whether a promise was made to pay for hostages, whether there was anything that could be deemed a bribe for foreign officials, whether there was a quid pro quo,” the professor added. “What did this individual expect in return? This is not someone who is known for doing pro-bono work. He has close ties to the Russian government. He has some rather shady associations. He doesn’t put himself at risk, use his time, and put forward a lot of money unless he expects something in return.”

Flash forward to September of 2016, when three FBI agents visited Deripaska’s apartment when he was in New York as part of Russia’s United Nations delegation. At least one agent had reportedly worked with Deripaska on the failed effort to rescue Levinson.

During the hour-long visit, the agents allegedly pitched the narrative that Trump’s campaign was secretly colluding with Russia to hijack the U.S. election. On Fox News’ The Ingraham Angle Monday night, Solomon pointed out that the agents had specifically brought up Paul Manafort’s name — a man with whom Deripaska had had a business relationship in the past until they’d had a bitter falling out.

“So the guy they go to is an enemy of Paul Manafort, and they say ‘well,can you help us prove that maybe Manafort is colluding with Putin to throw the whole election to Donald Trump?'” Solomon said.

According to Deripaska’s lawyer, his client “laughed but realized, despite the joviality, that they were serious. So he told them in his informed opinion the idea they were proposing was false. ‘You are trying to create something out of nothing,’ he told them.”

Solomon told host Laura Ingraham that the agents urged Deripaska to “keep an open mind and stay in touch.”

Solomon pointed out that it might be problematic if the FBI had failed tell the FISA court that one of its past Russian sources laughed off their Trump-Russia collusion theories.

He also noted: “The U.S. government in April imposed sanctions on Deripaska, one of several prominent Russians targeted to punish Vladimir Putin, using the same sort of allegations that State used from 2006 to 2009. Yet, between those two episodes, Deripaska seemed good enough for the FBI to ask him to fund that multimillion-dollar rescue mission. And to seek his help on a sensitive political investigation. And to allow him into the country eight times.”

Senator Mark Warner, the top Democrat on the Senate Intelligence Committee, communicated through phone calls and text messages with Deripaska’s lobbyist Adam Waldman in March and April of 2017. It was an effort to gain access to British spy Christopher Steele, and to information that could “help our country.” In one text Warner expressed a desire to “not have a paper trail,” and he suggested in another text that he did not want any other senator included in the discussions.

It is not known if Deripaska has been in contact with the FBI since September 2016.

“I also understand that Deripaska offered to testify on Capitol Hill,” Ingraham said, noting that he did not ask for immunity according to her two sources.

“It was reported by the New York Times — falsely — that he asked for immunity to testify,” she said. [NYT story here.] “He did not ask for immunity. Why are they smearing this guy?” Ingraham asked.

Lawmakers reportedly declined to accept Deripaska’s conditions, so he will not be testifying before any committees.