Carter Page Sergei Karpukhin/Reuters

The FBI extensively questioned Carter Page, an early foreign policy adviser to President Donald Trump's campaign, in March about allegations that he served as a middleman between the campaign and Moscow during the election, Page confirmed to Business Insider on Monday.

Those allegations were laid out in an explosive but unverified collection of memos, now known as the Trump-Russia dossier, accusing the Trump campaign of colluding with Russia to undermine Democratic presidential nominee Hillary Clinton during the election.

The memos were written by Christopher Steele, a former British spy who spent years working in Moscow. Steele wrote the memos for Fusion GPS, an opposition research firm based in Washington, DC that was hired by never-Trump Republicans to dig up dirt on the president in the early days of his candidacy.

The dossier alleges that Page was part of a "well-developed conspiracy of cooperation between (Trump associates) and the Russian leadership." It alleges that Igor Sechin, the CEO of Russia's state oil company Rosneft, offered Page the brokerage of a 19% stake in the company in exchange for the lifting of US sanctions on Russia when Page was in Moscow last July.

While there, the dossier alleges, Page also met with senior Kremlin internal affairs official Igor Diveykin, whom US officials believe was responsible for the intelligence collected by Russia about the US election. Page traveled to Moscow again in December to meet with Rosneft officials, he told reporters at the time.

Page denied all of the allegations throughout five separate interviews and more than 10 hours of questioning from the FBI, according to The Washington Post, which first broke the news. He has called accusations that he served as a liaison an "illegal" form of "retribution" for his speech at the New Economic School last July, in which he slammed the US for a "hypocritical focus on ideas such as democratization, inequality, corruption, and regime change."

That the FBI questioned Page about accusations made in the dossier, however, indicates that the bureau is using the document as a "roadmap" for its investigation into Russia's interference in the 2016 election, as the BBC's Paul Wood reported in March.

'Successful collaboration' between Steele and the FBI

The FBI may be taking cues from Steele's dossier because it has worked with him in the past, according to Wood.

Steele, who cultivated an extensive network of Russian sources during his time on British intelligence agency MI6's Moscow desk, apparently worked with the FBI on Russia- and Ukraine-related matters between 2013 and 2016 - specifically with the FBI's Eurasian Joint Organized Crime Squad, according to a lengthy profile in Vanity Fair.

The squad "was a particularly gung-ho team with whom Steele had done some heady things in the past," Vanity Fair reported. "And in the course of their successful collaboration, the hard-driving FBI agents and the former frontline spy evolved into a chummy mutual-admiration society."

The relationship was so "chummy" that the FBI offered to pay Steele to continue his work in October, The Washington Post reported in February.

Some of the dossier's more outlandish claims, including salacious accounts of sexual escapades, have not been confirmed. Trump has dismissed the dossier as "phony stuff" and "fake news."

But comparing Steele's reports, which were written between June and December, with events that unfolded before and after the election reveals a series of coincidences that has added to questions surrounding Russia's interference in the election.

An 'informal' adviser

The FBI obtained a FISA (Foreign Intelligence Surveillance Act) warrant - and has renewed it more than once - to monitor Page's communications last summer after he traveled to Moscow.

Page told Business Insider when he learned of the FISA warrant that he "was so happy to hear that further confirmation [of surveillance] is now being revealed." He called the warrant "unjustified" and "illegal."

But the bureau's application for the warrant, The Washington Post reported earlier this year, "included a lengthy declaration that laid out investigators' basis for believing that Page was an agent of the Russian government and knowingly engaged in clandestine intelligence activities on behalf of Moscow."

There were contacts Page had with Russian intelligence officials that he did not disclose, according to the Post, and unanswered questions about a court case involving a Russian spy who tried to recruit him in 2013.

Page found himself at the center of a Russia-related firestorm in late February after USA Today reported that he met Russia's ambassador to the US, Sergey Kislyak, during an event at the Republican National Convention. At least two other Trump associates, former adviser J.D. Gordon and then-Sen. Jeff Sessions, now the US attorney general, also reportedly spoke with Kislyak at the convention.

The White House has distanced itself from Page, insisting that he was an "informal" adviser to the campaign who left in early September. Page had no badge, an administration official told Business Insider in May, and never signed a nondisclosure agreement - two requirements of anyone working with the campaign in an official capacity. He also wasn't on the campaign's payroll, the official said, and did not have a campaign email account.

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