Two US intelligence chiefs are set to testify before Congress about possible links between Russia and President Donald Trump's election campaign.
They will also address Mr Trump's unsubstantiated claim that he was wiretapped by predecessor Barack Obama, BBC reports.
FBI director James Comey and NSA chief Admiral Mike Rogers will give evidence at a rare open hearing of the congressional intelligence committee.
Mr Trump has called the investigation a "total witch hunt".
Russia denies attempting to influence the US presidential election.
What are the allegations?
In January, US intelligence agencies said Kremlin-backed hackers had broken into the email accounts of senior Democrats and released embarrassing ones in order to help Mr Trump defeat Hillary Clinton.
A report by the CIA, FBI and National Security Agency (NSA) said Russian leader Vladimir Putin "ordered" a campaign aimed at influencing the election.
Since then, Mr Trump has faced allegations that his campaign team had links to Russian officials.
Republican Devin Nunes, chairman of the House intelligence committee, and Adam Schiff, the panel's top Democrat, are leading an investigation into the allegations.
Mr Nunes said on Sunday that based on "everything I have up to this morning" there is "no evidence" that the Trump campaign colluded with Russia.
Former Director of National Intelligence James Clapper has also said he saw no evidence of any collusion.
However, Mr Schiff said the material he had seen offers circumstantial evidence that US citizens collaborated with Russians to influence the vote.
"There was circumstantial evidence of collusion; there is direct evidence, I think, of deception," Mr Schiff said. "There's certainly enough for us to conduct an investigation."