FBI director James Comey has confirmed for the first time that the FBI is investigating alleged Russian government interference in the 2016 election.
That includes investigating possible links between individuals in the Trump campaign and the Russian government and whether there was co-ordination between the Trump campaign and Russia, he said.
The FBI would also assess whether crimes were committed, he said.
Mr Trump has denied any collusion.
Russia has always denied attempting to influence the US presidential election.
The FBI director was speaking at a rare open hearing of the congressional intelligence committee, which is also investigating possible links between the Trump campaign and Russia.
Mr Comey said the investigation was “very complex” and said he could not give the committee details that were not already publicly known.
He also said he could not give a timetable for its completion.
“We will follow the facts wherever they lead,” he said.
Also testifying before the committee is National Security Agency (NSA) chief Admiral Mike Rogers.
He said the NSA stood by an intelligence community report published in January, which said that Russian President Vladimir Putin had ordered a campaign to harm the campaign of Mr Trump’s rival, Hillary Clinton.
Devin Nunes, the Republican chairman of the House of Representatives Intelligence Committee, denied a claim by Mr Trump that Trump Tower had been wiretapped.
Mr Trump had tweeted earlier this month that former president Barack Obama had ordered a wiretap.
“Let me be clear: We know there was not a wiretap on Trump Tower,” he said.
However it was still possible that other surveillance activities had been used against Mr Trump and his associates, he said.