Senator John McCain passed documents to the FBI director, James Comey, last month alleging secret contacts between the Trump campaign and Moscow and that Russian intelligence had personally compromising material on the president-elect himself.
The material, which has been seen by the Guardian, is a series of reports on Trump's relationship with Moscow. They were drawn up by a former western counter-intelligence official, now working as a private consultant.
The Guardian has not been able to confirm the veracity of the documents' contents, and the Trump team has consistently denied any hidden contacts with the Russian government.
Trump's transition team did not immediately respond to a request for comment, but late on Tuesday, Trump tweeted: "FAKE NEWS - A TOTAL POLITICAL WITCH HUNT!" He made no direct reference to the allegations.
An official in the US administration who spoke to the Guardian described the source who wrote the intelligence report as consistently reliable, meticulous and well-informed, with a reputation for having extensive Russian contacts.
Some of the reports - which are dated from 20 June to 20 October last year - also proved to be prescient, predicting events that happened after they were sent.
One report, dated June 2016, claims that the Kremlin has been cultivating, supporting and assisting Trump for at least five years, with the aim of encouraging "splits and divisions in western alliance".
It claims that Trump had declined "various sweetener real estate deals offered him in Russia" especially in developments linked to the 2018 World Cup finals but that "he and his inner circle have accepted a regular flow of intelligence from the Kremlin, including on his Democratic and other political rivals."
Most explosively, the report alleges: "FSB has compromised Trump through his activities in Moscow sufficiently to be able to blackmail him." The president-elect has not responded to the allegations.
CNN reported on Tuesday that the FBI was still investigating the credibility of the documents but added that the intelligence chiefs had included a summary of the material in a secret briefing on Russian interference in the election delivered last week to Barack Obama and Donald Trump.
The emergence of the documents is potentially explosive, 10 days before Trump's inauguration and on the eve of his first planned press conference since July last year.
Despite glowing references from US and foreign officials who have worked with the source, there are some errors in the reports. One describes the Moscow suburb of Barvikha as "reserved for the residences of the top leadership and their close associates", but although it is a very expensive neighbourhood, there are no restrictions on who can own property there. The document also misspells the name of a Russian banking corporation.
The FBI does not normally make any comment on ongoing counter-intelligence investigations but was under increasing pressure from Democrats and some Republicans to act before the inauguration, particularly because of Comey's announcement of a continuing investigation into Hillary Clinton's email server 11 days before the election, which many of her supporters believe cost her the presidency.
The reports were initially commissioned as opposition research during the presidential campaign, but its author was sufficiently alarmed by what he discovered to send a copy to the FBI. It is unclear who within the organisation they reached and what action the bureau took. The former Democratic Senate leader, Harry Reid, has lambasted Comey for publicising investigations into Hillary Clinton's private server, while allegedly sitting on "explosive" material on Trump's ties to Russia.(Source: The Guardian)