McCain - a longstanding anti-Trump Republican who had disassociated himself from the candidate's campaign weeks before the election - cast himself as an innocent and concerned member of the public as he justified his move.
He claimed he had no idea whether it was accurate or not - but that he believed the FBI should have it because it was 'sensitive'.
'I did what any citizen should do. I received sensitive information and handed it to the FBI,' he told CNN - the network which broke the story that the document existed. It was then published in full by Buzzfeed.
'That's why I gave it to the FBI. I don't know if it is credible or not but the information I thought deserved to be delivered to the FBI, the appropriate agency of government.'
He added: 'It doesn't trouble me because I don't know if it is accurate or not. I have no way of corroborating that.
'The individual gave me the information. I looked at it. After receiving that information I took it to the FBI.'
He added that he was now aware from media reports that the FBI was apparently already in possession of the information. '
The Arizona senator had issued a public statement amid mounting questions of his exact role in the affair - and how a document riddled with errors and unverifiable claims came to be published.
'Late last year, I received sensitive information that has since been made public,' he said.
'Upon examination of the contents, and unable to make a judgment about their accuracy, I delivered the information to the Director of the FBI.
'That has been the extent of my contact with the FBI or any other government agency regarding this issue.'
But the 2008 Republican loser, who disowned his party's candidate weeks before the election, may have been far more intimately involved than that.
However Carl Bernstein, the Watergate reporter who contributed to the first story about its existence, published by CNN on Tuesday afternoon, suggested that McCain was handed it by a former British ambassador to Moscow.
Bernstein told CNN: 'It came from a former British MI6 agent who was hired from a political opposition research firm in Washington who was doing work about Donald Trump for both republican and democratic candidates opposed to Trump.
'They were looking at Trumps business ties, they saw some questionable things about Russians, about his businesses in Russia, they in turn hired this MI6 former investigator, he then came up with additional information from his Russian sources, he was very concerned by the implications of it, he then took it to an FBI colleague that he had known in his undercover work for years, he took it to this FBI man in Rome who turned it over to the bureau in Washington in August.
'And then, a former British ambassador to Russia independently was made aware of these findings and he took the information to John McCain - Senator John McCain of Arizona - in the period just after the election, and showed it to McCain - additional findings.
'McCain was sufficiently disturbed by what he read to take it to FBI director James Comey himself personally, they had a five minute meeting the two men, very little was said, McCain turned it over to him and is now awaiting what the FBI's response is to that information.'
The identity of the former British ambassador has not been disclosed.
Only one former British ambassador to Moscow remains in UK government service, Sir Tim Barrow, who went on to be Foreign Office political director and is now Britain's ambassador to the European Union.
McCain's long-standing opposition to Trump is well known although he only formally ended support for the Republican candidate in October, when the notorious 'p****' tape emerged.
The Arizona senator said at the time: '"Donald Trump's behavior... concluding with the disclosure of his demeaning comments about women and his boasts about sexual assaults, make it impossible to continue to offer even conditional support for his candidacy.'
US intelligence agencies have claimed that Russian spies hacked the Democratic National Committee and leaked damaging emails designed to undermine Hillary Clinton's campaign for president.
The new information, which has not been independently verified, claims that Russian officials also gathered highly damaging information on Trump, but only released the details attacking Clinton through the WikiLeaks website.
The Kremlin has denied all of the allegations, while Trump tweeted: 'FAKE NEWS - A TOTAL POLITICAL WITCH HUNT'.
However, McCain was so concerned about the information contained within the 35-page dossier, which included allegations that Trump had hired prostitutes in Moscow to urinate on a bed that had previously been used by US President Barack Obama and his wife Michelle, that he passed the information onto the FBI
CNN reported that intelligence chiefs had presented Trump with a two-page summary of the dossier late last week following a briefing with President Obama.
It is not known if it included the most salacious details.
Trump has consistently denied that Russian intelligence agencies had launched a massive cyber attack ahead of last November's election.
The dossier which McCain passed to FBI Director James Comey was compiled by the former MI6 man.
The memos describe sex videos involving prostitutes filmed during a 2013 visit by Trump to a luxury Moscow hotel, supposedly as a potential means for blackmail.
They also suggest Russian officials proposed lucrative deals in order to win influence over the Republican real estate magnate.
One claim, that special counsel to Trump Michael Cohen met with Kremlin officials in Prague in August 2016 has been branded as 'fake news'.
Cohen denied that he was central 'to the ongoing secret liaison relationship between the New York tycoon's campaign and the Russian leadership'.
Cohen tweeted a photograph of his passport and said he had never visited Prague.
According to reports, the former MI6 man had been hired to conduct 'opposition research' on the Trump campaign by first Republican enemies of Trump, then Democratic ones.
Russia denied the claims, with President Vladimir Putin's spokesman Dmitry Peskov telling journalists: 'The Kremlin does not have compromising information on Trump.'
The Kremlin spokesman called the dossier a 'total fake' and 'an obvious attempt to harm our bilateral relations'.
Earlier, the Kremlin had denied hacking the Democratic National Committee and leaking information to deliberately weaken Hillary Clinton's campaign.
Donald Trump's transition team has repeatedly denied allegations that it had received any help from Moscow.
The 35-page document, which had been collated by a former British Secret Intelligence Service agent had initially been handed over to the FBI in August.
Buzzfeed's Editor-in-Chief Ben Smith wrote an email to his staff shortly after publishing the document.
He justified the decision to release the information to allow Americans 'to make up their own minds about the allegations' which had been circulated at the 'highest levels of the US government'.
Comment: In most civilised countries, that is called slander and would result in the journalist being sued.
Smith claimed publishing the dossier was an example of 'transparent' journalism.
Further down in his memo, he appeared to discredit his own story as 'there is serious reason to doubt the allegations'.
He said Buzzfeed had been trying to stand up the allegations for 'weeks' - suggesting that it had failed to do so. Both the New York Times and the Washington Post said they had been handed the material and been unable to verify it.