FBI Director James Comey has confirmed that the bureau is investigating possible Russian interference in the 2016 presidential election, including alleged links between Moscow and Donald Trump's campaign.
Testifying before the House Select Committee on Intelligence, Comey said the probe is part of the FBI's counter-intelligence mission, and "includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia's efforts."
He declined to elaborate, citing an "open ongoing investigation" which is "classified."
Comey said he is authorized by the Justice Department to make the disclosure, although the FBI does not typically discuss or confirm the existence of ongoing investigations.
The US government previously accused Russia of hacking the Democratic Party's computer networks during the 2016 election, alleging that Moscow was attempting to "interfere" with the results – an allegation which the Kremlin has vehemently denied as untrue and baseless.
For his part, Trump has also said he has no knowledge of his associates coordinating with Russia during the election.
Trump slammed the investigation in a series of tweets earlier on Monday, calling the story "fake news" and alleging that it was "made up" by Democrats.
He went on to say that the "real story" that the FBI and Congress should be looking into is the leaking of classified information.
On Sunday, House Intelligence Committee Chair Devin Nunes reiterated to Fox News that "no evidence of collusion" between Trump's team and Russia had been found ahead of the first public meeting on the matter.
Nunes also addressed Trump's allegation that Obama had wiretapped Trump Tower, stating that no evidence had been found in that matter either.
Meanwhile, Comey was also questioned over Trump's claim that former President Barack Obama ordered surveillance of the billionaire's communications in Trump Tower during the campaign.
He stated that he has "no information that supports" the allegation, adding that any electronic surveillance must be granted by the courts, and that "no individual in the United States can direct electronic surveillance of anyone."