In the estimation of US officials, Oleg Deripaska, a Russian oligarch with close ties to the Kremlin, has faced credible accusations of extortion, bribery and even murder.

They also thought he might make a good source, so between 2014 and 2016, the FBI and the Justice Department unsuccessfully tried to turn him into an informant.

They signalled that they might provide help with his trouble in getting visas for the United States or even explore other steps to address his legal problems. In exchange, they were hoping for information on Russian organised crime and, later, on possible Russian aid to President Donald Trump’s 2016 campaign, according to current and former officials and associates of Mr Deripaska.

In one dramatic encounter, FBI agents appeared unannounced and uninvited at a home Mr Deripaska maintains in New York and pressed him on whether Paul Manafort, a former business partner of his who went on to become chairman of Mr Trump’s presidential campaign, had served as a link between it and the Kremlin.

The attempt to flip Mr Deripaska was part of a broader, clandestine US effort to gauge the possibility of gaining cooperation from roughly a half-dozen of Russia’s richest men, nearly all of whom, like Mr Deripaska, depend on President Vladimir Putin to maintain their wealth, the officials said.

Two of the players in the effort were Bruce Ohr, the Justice Department official who has recently become a target of attacks by Mr Trump and Christopher Steele, the former British spy who compiled a dossier of purported links between the Trump campaign and Russia.

The systematic effort to win the cooperation of the oligarchs, which has not previously been revealed, does not appear to have scored any successes. And in Mr Deripaska’s case, he told the US investigators that he disagreed with their theories about Russian organised crime and Kremlin collusion in the campaign, a person familiar with the exchanges said. The person added that Mr Deripaska even notified the Kremlin about the US efforts to cultivate him.

Oleg Deripaska, president of Russian aluminium giant Rusal, looks on before a meeting with Russian President Vladimir Putin and Turkish President Tayyip Erdogan (REUTERS)

But the fallout from the efforts is now rippling through US politics and has helped fuel Mr Trump’s campaign to discredit the investigation into whether he coordinated with Russia in its interference in the election.

The contacts between Mr Ohr and Mr Steele were detailed in emails and notes from Mr Ohr that the Justice Department turned over to Republicans in Congress earlier this year. A number of journalists, including some at conservative news outlets, have reported on elements of those contacts but not on the broader outreach program to the oligarchs or key aspects of the interactions between Mr Ohr, Mr Steele and Mr Deripaska.

The revelation that Mr Ohr engaged with Mr Steele has provided the president’s allies with fresh fodder to attack the investigation led by Robert Mueller, the special counsel, casting it as part of a vast, long-running conspiracy by a “deep state” bent on undermining Mr Trump. In their telling, Mr Ohr and his wife — who worked as a contractor at the same research firm that produced the dossier — are villainous central players in a cabal out to destroy the president.

Mr Trump himself has seized on the reports, threatening to pull Mr Ohr’s security clearance and claiming that his family “received big money for helping to create the phony, dirty and discredited Dossier.”

While Mr Steele did discuss the research that resulted in the dossier with Mr Ohr during the final months of the campaign, current and former officials said that Mr Deripaska was the subject of many of the contacts between the two men between 2014 and 2016.

A timeline that Mr Ohr hand-wrote of all his contacts with Mr Steele was among the leaked documents cited by the president and his allies as evidence of an anti-Trump plot.

The contacts between Mr Steele and Mr Ohr started before Mr Trump became a presidential candidate and continued through much of the campaign.

Mr Deripaska’s contacts with the FBI took place in September 2015 and the same month a year later. The latter meeting came two months after the FBI began investigating Russian interference in the election and a month after Manafort left the Trump campaign amid reports about his work for Russia-aligned political parties in Ukraine.

Mr Steele sought to aid the effort to engage Mr Deripaska, and he noted in an email to Mr Ohr in February 2016 that the Russian had received a visa to travel to the United States. In the email, Mr Steele said his company had compiled and circulated “sensitive” research suggesting that Mr Deripaska and other oligarchs were under pressure from the Kremlin to toe the Russian government line, leading Mr Steele to conclude that Mr Deripaska was not the “tool” of Putin alleged by the US government.

The timeline sketched out by Mr Ohr shows contacts stretching back to when Mr Ohr first met Mr Steele in 2007. It also shows what officials said was the first date on which the two discussed cultivating Mr Deripaska: a meeting in Washington on November 21, 2014, roughly seven months before Mr Trump announced that he was running for president.

Paul Ryan advocates sanctions on Russia: It is a 'menacing government'

The officials spoke on the condition of anonymity to discuss an initiative that remains classified. Most expressed deep discomfort, saying they feared that in revealing the attempts to cultivate Mr Deripaska and other oligarchs they were undermining US national security and strengthening the grip that Mr Putin holds over those who surround him.

But they also said they did not want Mr Trump and his allies to use the program’s secrecy as a screen with which they could cherry-pick facts and present them, sheared of context, to undermine the special counsel’s investigation. That, too, they said they feared, would damage US security.

The program was led by the FBI. Mr Ohr, who had long worked on combating Russian organised crime, was one of the Justice Department officials involved.

Mr Steele served as an intermediary between the Americans and the Russian oligarchs they were seeking to cultivate. He had first met Mr Ohr years earlier while still serving at MI6, Britain’s foreign spy agency, where he oversaw Russia operations. After retiring, he opened a business intelligence firm, and had tracked Russian organised crime and business interests for private clients, including one of Mr Deripaska’s lawyers.

To facilitate meetings, the FBI pushed the State Department to allow Mr Deripaska to travel to New York on a Russian diplomatic passport as part of a Russian government delegation to the UN General Assembly. The State Department had previously rejected some of Mr Deripaska’s efforts to secure visas to enter the United States — even as part of prior diplomatic delegations — but it approved diplomatic visa requests in 2015 and 2016.

Mr Steele helped set up a meeting between the Russian and US officials during the 2015 trip. Mr Ohr attended the meeting, during which the Americans pressed Mr Deripaska on the connections between Russian organised crime and Mr Putin’s government, as well as other issues, according to a person familiar with the events. The person said that Mr Deripaska told the Americans that their theories were off base and did not reflect how things worked in Russia.

Mr Deripaska would not agree to a second meeting. But one took place the following year, in September 2016, when FBI agents showed up unannounced at his door in New York. By then, they were already investigating possible ties between Russia and the Trump campaign and they pressed Mr Deripaska about whether his former business partner, Manafort, had served as a link to the Kremlin during his time as Mr Trump’s campaign chairman.

It was not only the FBI that was concerned about Russian interference in the final months of the campaign. US spy agencies were sounding an alarm after months of intelligence reports about contacts between Trump associates and Russians and Moscow’s hacking of Democratic Party emails. (US intelligence agencies would later conclude that the interference was real and that Russia had acted to boost Mr Trump’s candidacy.)

There was also a growing debate at the highest levels of the Obama administration about how to respond without being seen as trying to tip the presidential election towards Hillary Clinton.

Mr Deripaska, though, told the FBI agents that while he had no love for Manafort, with whom he was in a bitter business dispute, he found their theories about his role on the campaign “preposterous.” He also disputed that there were any connections between the Trump campaign and Russia, according to the person familiar with the exchange.

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The Justice Department’s efforts to cultivate Mr Deripaska appear to have fizzled soon after, amid worsening relations between the United States and Russia.

This past April, the Treasury Department imposed potentially crippling sanctions against Mr Deripaska and his mammoth aluminium company, saying he had profited from the “malign activities” of Russia around the world. In announcing the sanctions, the Trump administration cited accusations that Mr Deripaska had been accused of extortion, racketeering, bribery, links to organised crime and even ordering the murder of a businessman.

Mr Deripaska has denied the allegations and his allies contend that the sanctions are punishment for refusing to play ball with the Americans.

Even after the concerted effort to cultivate Mr Deripaska appeared to have broken down, and as he was emerging as a subject of increasing interest in inquiries into ties between Mr Trump’s circle and Russia, both sides continued sporadic outreach.

Last year, Mr Ohr asked someone who communicated with Mr Deripaska to urge the oligarch to “give up Manafort,” according to a person familiar with the exchange.

And Mr Deripaska sought to engage with Congress.

The oligarch took out newspaper advertisements in the United States last year volunteering to testify in any congressional hearings examining his work with Manafort. The ads were in response to an Associated Press report that Manafort had secretly worked for Mr Deripaska on a plan to “greatly benefit the Putin government” in the mid-2000s.

Mr Deripaska deplored that assertion as “malicious” and a “lie,” and subsequently sued AP for libel, though he later dropped his appeal of a judge’s ruling dismissing the lawsuit without receiving a settlement or payment.

Soon after the advertisements ran, representatives for the House and Senate Intelligence committees called a Washington-based lawyer for Mr Deripaska, Adam Waldman, inquiring about taking his client up on the offer to testify, Mr Waldman said in an interview.

What happened after that has been in dispute. Mr Waldman, who stopped working for Mr Deripaska after the sanctions were levied, said he told the committee staff that his client would be willing to testify without any grant of immunity, but would not testify about any Russian collusion with the Trump campaign because “he doesn’t know anything about that theory and actually doesn’t believe it occurred.”

“I told them that he would be willing to talk about Manafort,” Mr Waldman added.

Mr Waldman said he did not hear back from the committee’s staff members, but he contends that they played a role in pushing the claim that the talks over Mr Deripaska’s potential testimony had fallen apart because he demanded immunity.

“We specifically told them that we did not want immunity,” Mr Waldman said. “Clearly, they did not want him to testify. What other conclusion could you possibly draw?”