Special counsel Robert Mueller has withstood relentless political attacks, many distorting his record of distinguished government service.

But there's one episode even Mueller's former law enforcement comrades - and independent ethicists - acknowledge raises legitimate legal issues and a possible conflict of interest in his overseeing the Russia election probe.

In 2009, when Mueller ran the FBI, the bureau asked Russian oligarch Oleg Deripaska to spend millions of his own dollars funding an FBI-supervised operation to rescue a retired FBI agent, Robert Levinson, captured in Iran while working for the CIA in 2007.

Yes, that's the same Deripaska who has surfaced in Mueller's current investigation and who was recently sanctioned by the Trump administration.

The Levinson mission is confirmed by more than a dozen participants inside and outside the FBI, including Deripaska, his lawyer, the Levinson family and a retired agent who supervised the case. Mueller was kept apprised of the operation, officials told me.

Some aspects of Deripaska's help were chronicled in a 2016 book by reporter Barry Meier, but sources provide extensive new information about his role.

They said FBI agents courted Deripaska in 2009 in a series of secret hotel meetings in Paris; Vienna; Budapest, Hungary, and Washington. Agents persuaded the aluminum industry magnate to underwrite the mission. The Russian billionaire insisted the operation neither involve nor harm his homeland.

"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.

Then in fall 2010, the operation secured an offer to free Levinson. The deal was scuttled, however, when the State Department become uncomfortable with Iran's terms, according to Deripaska's lawyer and the Levinson family.

FBI officials confirmed State hampered their efforts.

"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.

"Deripaska's efforts came very close to success," said David McGee, a former federal prosecutor who represents Levinson's family. "We were told at one point that the terms of Levinson's release had been agreed to by Iran and the U.S. and included a statement by then-Secretary of State Hillary Clinton pointing a finger away from Iran. At the last minute, Secretary Clinton decided not to make the agreed-on statement."

The State Department declined comment, and a spokesman for Clinton did not offer comment. Mueller's spokesman, Peter Carr, declined to answer questions. As did McCabe.

The FBI had three reasons for choosing Deripaska for a mission worthy of a spy novel. First, his aluminum empire had business in Iran. Second, the FBI wanted a foreigner to fund the operation because spending money in Iran might violate U.S. sanctions and other laws. Third, agents knew Deripaska had been banished since 2006 from the United States by State over reports he had ties to organized crime and other nefarious activities. He denies the allegations, and nothing was ever proven in court.

The FBI rewarded Deripaska for his help. In fall 2009, according to U.S. entry records, Deripaska visited Washington on a rare law enforcement parole visa. And since 2011, he has been granted entry at least eight times on a diplomatic passport, even though he doesn't work for the Russian Foreign Ministry.

Former FBI officials confirm they arranged the access.

Deripaska said in a statement through Adam Waldman, his American lawyer, that FBI agents told him State's reasons for blocking his U.S. visa were "merely a pretext."

"The FBI said they had undertaken a careful background check, and if there was any validity to the State Department smears, they would not have reached out to me for assistance," the Russian said.

Then, over the past two years, evidence emerged tying him to former Trump campaign chairman Paul Manafort, the first defendant charged by Mueller's Russia probe with money laundering and illegal lobbying.

Deripaska once hired Manafort as a political adviser and invested money with him in a business venture that went bad. Deripaska sued Manafort, alleging he stole money.

Mueller's indictment of Manafort makes no mention of Deripaska, even though prosecutors have evidence that Manafort contemplated inviting his old Russian client for a 2016 Trump campaign briefing. Deripaska said he never got the invite and investigators have found no evidence it occurred. There's no public evidence Deripaska had anything to do with election meddling.

Deripaska also appears to be one of the first Russians the FBI asked for help when it began investigating the now-infamous Fusion GPS "Steele Dossier." Waldman, his American lawyer until the sanctions hit, gave me a detailed account, some of which U.S. officials confirm separately.

Two months before Trump was elected president, Deripaska was in New York as part of Russia's United Nations delegation when three FBI agents awakened him in his home; at least one agent had worked with Deripaska on the aborted effort to rescue Levinson. During an hour-long visit, the agents posited a theory that Trump's campaign was secretly colluding with Russia to hijack the U.S. election.

"Deripaska laughed but realized, despite the joviality, that they were serious," the lawyer said. "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." The agents left though the FBI sought more information in 2017 from the Russian, sources tell me. Waldman declined to say if Deripaska has been in contact with the FBI since Sept, 2016.

So why care about some banished Russian oligarch's account now?

Two reasons.

First, as the FBI prepared to get authority to surveil figures on Trump's campaign team, did it disclose to the Foreign Intelligence Surveillance Court that one of its past Russian sources waived them off the notion of Trump-Russia collusion?

Second, 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.

I was alerted to Deripaska's past FBI relationship by U.S. officials who wondered whether the Russian's conspicuous absence from Mueller's indictments might be related to his FBI work.

They aren't the only ones.

Harvard law professor Alan Dershowitz told me he believes Mueller has a conflict of interest because his FBI previously accepted financial help from a Russian that is, at the very least, a witness in the current probe.

"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.

Melanie Sloan, a former Clinton Justice Department lawyer and longtime ethics watchdog, told me a "far more significant issue" is whether the earlier FBI operation was even legal: "It's possible the bureau's arrangement with Mr. Deripaska violated the Antideficiency Act, which prohibits the government from accepting voluntary services."

George Washington University constitutional law professor Jonathan Turley agreed: "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."

Now that sources have unmasked the Deripaska story, time will tell whether the courts, Justice, Congress or a defendant formally questions if Mueller is conflicted.

In the meantime, the episode highlights an oft-forgotten truism: The cat-and-mouse maneuvers between Moscow and Washington are often portrayed in black-and-white terms. But the truth is, the relationship is enveloped in many shades of gray.

John Solomon is an award-winning investigative journalist whose work over the years has exposed U.S. and FBI intelligence failures before the Sept. 11 attacks, federal scientists' misuse of foster children and veterans in drug experiments, and numerous cases of political corruption. He is The Hill's executive vice president for video.

[Editor's note: This post was updated at 8:10 p.m. on May 14, 2018.]