FBI director James Comey and NSA chief Admiral Mike Rogers are set to testify before Congress about possible links between Russia and President Donald Trump’s election campaign.
The two intelligence chiefs will also address Donald Trump’s unsubstantiated claim that he was wiretapped by President Barack Obama.
James Comey and Mike Rogers will give evidence at a rare open hearing of the congressional intelligence committee.
President Trump has called the investigation a “total witch hunt”.
Russia denies attempting to influence the US presidential election.
Two months ago, US intelligence agencies said Kremlin-backed hackers had broken into the email accounts of senior Democrats and released embarrassing ones in order to help Donald Trump defeat rival Hillary Clinton.
A CIA, FBI and NSA report said Russian President Vladimir Putin “ordered” a campaign aimed at influencing the election.
Since then, Donald 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.
Devin Nunes said on March 19 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, Adam Schiff said the material he had seen offers circumstantial evidence that US citizens collaborated with Russians to influence the vote.
He said: “There was circumstantial evidence of collusion; there is direct evidence, I think, of deception.
“There’s certainly enough for us to conduct an investigation.”
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.
Michael 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, Jeff Sessions was accused by Democrats of lying under oath during his confirmation hearing in January.
Jeff Sessions said he had “no communications with the Russians”, but it later emerged that he had met Sergei Kislyak during the campaign.
He denied any wrongdoing, but removed himself from an FBI inquiry into Russia’s alleged interference in the election.
March 20 hearing is also expected to address Donald Trump’s claims that the Obama administration wiretapped his phone at Trump Tower in New York during the campaign.
President Trump has provided no evidence, and senior Republican and Democratic officials have dismissed the idea. Barack Obama’s spokesman dismissed the claims.
Devin Nunes told Fox News on March 19 that a review of justice department documents provided on March 17 indicated there was no such wiretap.
Several Republicans have said President Trump should apologize if he cannot substantiate his claims.
Observers say both allegations have diverted attention from the Trump administration’s other policies and progress with political appointments.
Critics say Donald Trump’s claim that Barack Obama wiretapped him has damaged the US credibility, and relations with its allies.
Last week, President Trump’s spokesman Sean Spicer repeated claims by a Fox News analyst that the UK’s GCHQ spy agency had helped Barack Obama wiretap Donald Trump.
The claims angered the UK government, and GCHQ rejected the allegations as “utterly ridiculous”.
Meanwhile, President Trump and some Republicans have called for an investigation into intelligence leaks, including the leak that revealed details of Michael Flynn’s phone calls to the Russian ambassador.