The FBI investigators treated Steele as a peer -- a Russia expert so well-trusted that he had assisted the Justice Department on past cases and provided briefing material for British prime ministers and at least one U.S. president. During intense questioning that day in Rome, they alluded to some of their own findings of ties between Russia and the Trump campaign and raised the prospect of paying Steele to continue gathering intelligence after Election Day, according to people familiar with the discussion.

But Steele was not one of them. He had left the Secret Intelligence Service, or MI6, seven years earlier and was now working on behalf of Fusion GPS, a private Washington research firm whose work at the time was funded by Trump's opponent, Hillary Clinton, and the Democratic Party.

The meeting in Rome captured the unusual and complicated role of Steele, who wrote memos that came to be known as the dossier and who has become the central point of contention in the political brawl raging around the Russia inquiry by special counsel Robert S. Mueller III.

Those who believe Steele consider him a hero, a latter-day Paul Revere who, at personal risk, tried to provide an early warning about the Kremlin's unprecedented meddling in a U.S. campaign. Those who distrust him say he is merely a hired gun leading a political attack on Trump.

Steele himself struggled to navigate dual obligations -- to his private clients, who were paying him to help Clinton win, and to a sense of public duty born of his previous life.

Sir Andrew Wood, a British former diplomat and friend of Steele, said he urged him in the fall of 2016 to alert the authorities. "The right sort of people" needed to be told, Wood said he told Steele. "My opinion was, 'You don't have a choice. At least, you don't have an honorable choice.' " [...]

He was steeped in Russia early on after being recruited to Britain's elite spy service from Cambridge University. He spent two decades working for the famed MI6 spy agency, including a stint in his mid-20s in Moscow, where he served undercover in the British Embassy.

When he returned to work for the agency in London, he provided briefing materials on Russia for senior government officials and led the British inquiry into the mysterious 2006 death in London of Alexander Litvinenko, a former KGB official and Putin critic.

In 2009, after more than two decades in public service, Steele turned to the private sector and founded a London-based consulting firm, Orbis Business Intelligence, drawing on the reputation and network he developed doing intelligence work.

Among those who have continued to seek his expertise is Steele's former boss Richard Dearlove, who headed MI6 from 1999 to 2004.

In an interview, Dearlove said Steele became the "go-to person on Russia in the commercial sector" following his retirement from the Secret Intelligence Service. He described the reputations of Steele and his business partner, fellow intelligence veteran Christopher Burrows, as "superb."

 Sir Richard Dearlove, a former head of MI6, called Steele the "go-to person on Russia." (Stefan Rousseau/PA Wire via Associated Press)

In one of his first cases as a private consultant, Steele worked closely with the FBI in its investigation of corruption at FIFA, the powerful worldwide soccer governing body. Steele, who at the time was working for the English Football Association, shared his research with top officials at the Justice Department. U.S. officials eventually charged 14 top soccer executives and their associates with wire fraud, racketeering and money laundering.

Steele and Burrows soon amassed a group of clients that included multinational companies and wealthy business titans, including some Russians, according to people familiar with their work.

Steele continued to feed information to the U.S. government, passing along intelligence he gathered about Ukraine and Russia for corporate clients in 2014 and 2015 to a friend at the State Department, according to former assistant secretary of state Victoria Nuland. "He offered us that reporting free, so that we could also benefit from it," she said Sunday on CBS's "Face the Nation."

In June 2016, Steele was contacted by Glenn Simpson, a former Wall Street Journal reporter and co-founder of Fusion GPS. Simpson and Steele had been introduced by a mutual friend in 2009 who knew that they shared a near-obsessive interest in Russian organized crime and that they had worked together on previous cases.

Now Simpson had an intriguing offer: Would Steele's firm help research Trump's ties to Russia? [...]

Most of Simpson's research was based on scouring public records, court filings and media reports from around the world.

Steele brought far more: He was able to tap a network of human sources cultivated over decades of Russia work. He moved quickly, reaching out to Russian contacts and others he referred to as "collectors" who had other sources -- some of whom had no idea their comments would be passed along to Steele.

His sources included "a close associate of Trump," as well as "a senior Russian foreign ministry figure" and a "former top-level Russian intelligence officer," both of whom Steele indicated had revealed their information to a "trusted compatriot," he later reported to Fusion GPS.

Just weeks after taking the case, Steele told friends that the initial intelligence he had gathered was "hair-raising." [...]

Steele told Simpson of his plan to meet with the FBI, describing it as an obligation rooted in his past work for the British government.

" 'I'm a former intelligence officer, and we're your closest ally,' " Steele told Simpson, according to testimony Simpson later gave to the House Intelligence Committee. " 'You know, I have obligations, professional obligations. If there's a national security emergency or possible national security issue, I should report it.' "

Simpson said he did not question Steele's judgment: "He's the spy," Simpson said. "I'm the ex-journalist."  [...]

In late July, Steele told friends he was rattled when WikiLeaks released thousands of internal Democratic National Committee emails on the eve of the Democratic National Convention, material that U.S. law enforcement officials said was hacked by Russia. Then Trump -- who had repeatedly praised Putin on the campaign trail -- publicly called on Russia to hack and release a cache of missing Clinton emails.

Steele, who had researched Russian attempts to interfere in European elections for another client, began to fear that the Americans were not taking the Kremlin's efforts seriously enough, associates said.