For over one year, Special Counsel Robert Mueller has investigated the alleged “Russian collusion” by the 2016 presidential campaign of Donald Trump. As well, Mueller has withstood political attacks, many of which have attempted to derail his lengthy career of government service. Nevertheless, there is one episode in the saga that even those close to Mueller and other ethicists acknowledge as a legitimate legal issue and possible conflict of interest of his leading the Russia probe.

An investigative report from The Hill outlines a new reality inside the Russian investigation. It tells the tale of a decade-old FBI operation that involved Mueller’s FBI, a secret Iranian mission, and a Russian billionaire.

When Mueller was deputy director of the FBI in 2009, the agency was attempting an operation to rescue Robert Levinson, a retired FBI agent, who was captured in Iran in 2007 while working for the CIA. The bureau asked Russian oligarch Oleg Deripaska to spend millions of his own dollars to fund the FBI-supervised operation.

Some may say they have heard the name Oleg Deripaska before – and they would be right. That is the same Deripaska who has surfaced in the Russia investigation and who was recently sanctioned by the Trump Administration.

Before the story proceeds, the Levinson mission, according to the report, has been confirmed by over a dozen “participants” both inside and outside the FBI. Deripaska, his lawyer, the family of Levinson, and the retired FBI agent that led the mission all confirmed the case, with Mueller being kept abreast of the proceedings.

In 2009, FBI agents courted Deripaska via a series of secret hotel meetings in Paris, Vienna, Budapest, Hungary, and Washington, D.C. Deripaska, an aluminum industry giant, was persuaded by the FBI to underwrite the mission, all with the promise that the billionaire’s operations and homeland remained unharmed.

“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 of the agents who was integral in helping persuade Deripaska was none other than Andrew McCabe. Yes, the same McCabe who played a seminal role in starting the Trump-Russia case and who was recently fired by President Trump just days before his retirement.

Deripaska’s lawyer said his client ultimately spent $25 million assembling a private search and rescue team that worked with Iranian contacts under the FBI’s watchful eye. Then in fall 2010, the operation secured an offer to free Levinson. However, the deal was abandoned when the State Department became uncomfortable with Iran’s terms, according to Deripaska’s lawyer and the Levinson family.

The report explained that FBI officials confirmed that the State Department, led by Hillary Clinton at the time, 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.”

The operation was officially disbanded in 2011 after the FBI was concerned that Deripaska’s Iranian contacts were unable to deliver results. Therefore, Levinson was never found and his whereabouts still remain a mystery.

“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 report states that the State Department, Clinton, her spokesman, Mueller, his spokesman Peter Carr, and McCabe all declined to answer the questions surrounding their involvement.

One may ask: why was Deripaska chosen to fund the mission?

The FBI has three reasons for doing so.

1. Deripaska’s aluminum empire has business dealings in Iran.

2. The FBI wanted a foreigner to underwrite the operation due to the fact that spending U.S. dollars in Iran would have violated the sanctions placed on the country by the U.S. at the time and possibly other laws.

3. Agents knew that Russian has been barred from entering the U.S. by the State Department since 2006 over ties to organized crime syndicates and other nefarious activities.

In all, 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. Moreover, since 2011, he has been granted entry at least eight times on a diplomatic passport, even though he does not work for the Russian Foreign Ministry.

The Hill spoke with former FBI officials who confirmed they arranged the access.

Deripaska provided a statement through Adam Waldman, his American lawyer, which said that FBI agents told him the State Department’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.

Since the end of the 2016 presidential election cycle, evidence emerged showing ties between Deripaska and former Trump Campaign Chairman Paul Manafort. The Russian once hired Manafort as a political adviser and invested money with him in a business venture that had less-then-stellar results. Deripaska then sued Manafort, alleging he stole money.

Interestingly, Mueller’s indictment of Manafort makes no mention of Deripaska, even though prosecutors have evidence that Manafort contemplated inviting his former Russian client for a 2016 Trump campaign briefing. Deripaska said he never got the invite and investigators have found no evidence it occurred.

Furthermore, there is no evidence that Deripaska had anything to do with election meddling.

Deripaska was also one of the first Russians the FBI asked for help when it began investigating the now-infamous Fusion GPS “Steele Dossier.”

In September 2016, two months before the presidential election, Deripaska was in New York as part of Russia’s UN delegation when three FBI agents awakened him in his home early in the morning. One of the agents present had worked with Deripaska on the aborted effort to rescue Levinson years before. During the visit, the agents posited the theory that the Trump campaign was secretly colluding with Russia in effort to win the election.

“Deripaska laughed but realized, despite the joviality, that they were serious,” his 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.”

It is unclear if the FBI has had any contact with Deripaska since that meeting, but the FBI has sought additional information on the Russian.

Why is the FBI seeking more information on Deripaska’s account now?

1. As the FBI prepared to get authority to conduct surveillance on members of Trump’s campaign team, question have been risen that they failed to disclose to the Foreign Intelligence Surveillance (FISA) Court that one of its past Russian sources waived them off the notion of Trump-Russia collusion.

2. In April, the Trump Administration placed sanctions on Deripaska – one of several oligarchs tied to Russian Vladimir Putin – using similar sanctions the State Department used previous to 2009. However, between the two episodes, Deripaska was valuable to the FBI, asking him to fund a multi-million dollar rescue mission, seek his assistance on a political investigation, and allow him diplomatic access into the U.S. at least eight times.

The Russian’s absence from Mueller indictments is precarious.

Harvard law scholar Alan Dershowitz says 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.

Former Clinton DOJ lawyer Melanie Sloan, a harsh ethics watchdog, says 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,” she said according to the report.

So, only time will tell if Mueller is formally questioned by the DOJ, Congress, or a defendant in the case about his conflict of interest in the investigation that has started to become a political witch hunt.