Two US intelligence chiefs are testifying before Congress over possible links between Russia and Donald Trump’s election campaign.
They will also address Mr Trump’s unsubstantiated claim that he was wiretapped by predecessor Barack Obama.
FBI director James Comey and NSA chief Admiral Mike Rogers are giving 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.
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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, up until the time he left his post in January.
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.”
Which campaign members have been accused of deception?
Two senior officials in the Trump administration have been caught up in the allegations – former national security adviser Michael Flynn, and Attorney-General Jeff Sessions.
Mr Flynn was fired last month after he misled the White House about his conversations with the Russian ambassador before he was appointed national security adviser.
He allegedly discussed US sanctions with ambassador Sergei Kislyak. It is illegal for private citizens to conduct US diplomacy.
Meanwhile, Mr Sessions was accused by Democrats of lying under oath during his confirmation hearing in January.
He said he had “no communications with the Russians”, but it later emerged that he had met Mr Kislyak during the campaign.
Mr Sessions denied any wrongdoing, but removed himself from an FBI inquiry into Russia’s alleged interference in the election.
What about Mr Trump’s claims against Mr Obama?
Monday’s hearing is also expected to address President Trump’s claims that the Obama administration wiretapped his phone at Trump Tower in New York during the campaign.
Mr Trump has provided no evidence, and senior Republican and Democratic officials have dismissed the idea. Mr Obama’s spokesman dismissed the claims.
Mr Nunes told Fox News on Sunday that a review of justice department documents provided on Friday indicated there was no such wiretap.
Several Republicans have said Mr Trump should apologise if he cannot substantiate his claims.
What impact have these claims had?
Observers say both allegations have diverted attention from the Trump administration’s other policies and progress with political appointments.
Critics say Mr Trump’s claim that Mr Obama wiretapped him has damaged the US credibility, and relations with its allies.
Last week, Mr Trump’s spokesman Sean Spicer repeated claims by a Fox News analyst that the UK’s GCHQ spy agency had helped Mr Obama wiretap Mr Trump.
The claims angered the UK government, and GCHQ rejected the allegations as “utterly ridiculous”.
Meanwhile, Mr Trump and some Republicans have called for an investigation into intelligence leaks, including the leak that revealed details of Mr Flynn’s phone calls to the Russian ambassador.
Trump Russia claims: US spy chiefs testify in Congress