FBI director James Comey has publicly confirmed for the first time that the agency is investigating Russia’s efforts to interfere in the 2016 US presidential election.
Mr Comey said this probe would include any potential coordination between Trump campaign associates and Russia’s government.
Testifying before US congress, Mr Comey said he is authorised by the American justice department to make the disclosure.
Typically, the FBI does not discuss or even confirm the existence of ongoing investigations.
Mr Comey said the probe is part of the FBI’s counter-intelligence mission.
He said the investigation includes the nature of any links between individuals associated with Mr Trump’s campaign and the Russian government, and whether there was any co-ordination between Russian efforts and the campaign.
Mr Comey said the investigation will also look at whether crimes were committed. He said he cannot provide details about the investigation.
Mr Comey told the US House Intelligence Committee: “This work is very complex, and there is no way for me to give you a timetable for when it will be done.”
Earlier in the hearing, the chairman of the committee contradicted an earlier assertion from Mr Trump by saying that there had been no wiretapping of Trump Tower.
However, representative Devin Nunes, a California Republican whose committee is one of several investigating, said that other forms of surveillance of Mr Trump and his associates have not been ruled out.
National Security Agency director Michael Rogers will also address the hearing later.
Mr Trump took to Twitter before the hearing began, accusing Democrats of making up allegations about his campaign associates’ contact with Russia during the election.
He said Congress and the FBI should be going after media leaks, and maybe even Hillary Clinton, instead.
Mr Trump tweeted: “The real story that Congress, the FBI and others should be looking into is the leaking of Classified information. Must find leaker now!”
Mr Trump also suggested, without evidence, that Mrs Clinton’s campaign was in contact with Russia and had possibly thwarted a federal investigation.
US intelligence officials have not publicly raised the possibility of contacts between the Clintons and Moscow.
Officials investigating the matter have said they believe Moscow had hacked into Democrats’ computers in a bid to help Mr Trump’s election bid.