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The FBI reached a deal in October to pay a former British spy who had compiled a dossier on Donald Trump's alleged ties with Russia, an indication of how seriously the bureau was taking the allegations, according to a person familiar with the matter.

The deal for the former operative, Christopher Steele, to continue his work on behalf of the FBI fell apart when Steele pulled out, said the source, who has direct knowledge of the situation.

The arrangement, first reported by The Washington Post, suggests that the FBI was trying hard to investigate Steele’s dossier, an assemblage of unverified and salacious claims that accuse the Trump campaign of colluding with Russia’s covert operation to interfere in the presidential election.

Steele began assembling the information on behalf of anti-Trump Republican clients, and then continued his research for Democrats.

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The FBI and Congress are investigating whether any Americans, including Trump associates, helped Russia in a campaign of hacking, leaking and fake news that U.S. intelligence agencies say was designed to sow chaos, harm Clinton, and help Trump. Trump aides deny that they colluded with the Russians.

An FBI spokesperson declined to comment.

Related: Christopher Steele, Trump Dossier Author, Is A Real-Life James Bond

A law enforcement official did not dispute the agreement to pay Steele, and noted that the FBI had a previous relationship with the former British intelligence operative. The source said Steele provided crucial help in the FBI’s investigation of corruption in FIFA, the international soccer body.

Steele first turned over information to the FBI in July, the source said, but heard little interest from the bureau. The FBI circled back in September, the source said, appearing to have renewed interest amid new hacks and leaks.

But Steele backed out of the deal before it was consummated, the source said.

Late last year, Sen. John McCain gave a copy of the dossier to FBI Director James Comey, McCain has said — and asked him whether any of it was true.

Related: Former British Spy Christopher Steele Prepared Explosive Trump Memo

After a January intelligence briefing, Comey informed Trump about the dossier, having already informed President Barack Obama.

Trump has said the allegations are entirely false, dismissing the dossier's contents as "fake news."

Foreign policy experts from both political parties have questioned why Trump has maintained an unusually favorable stance toward Russian President Vladimir Putin, an avowed U.S. adversary. Trump says he wants to find common ground with Russia to cooperate against terrorism, among other things.