FBI Director James Comey and NSA Director Admiral Mike Rogers have been called to testify as part of a congressional committee probe into Russian interference in the US presidential election.
The public could learn on Monday more of the extent to which Russia meddled in the US presidential campaign and whether that meddling included contact with Donald Trump.
The public could also finally learn whether there is any truth to the as yet unsubstantiated claim the US president has made that he was wiretapped by his predecessor, Barack Obama.
Representatives Devin Nunes, chairman of the House of Representatives Permanent Select Committee on Intelligence, and Adam Schiff, the panel's top Democrat, have called FBI Director James Comey and NSA Director Admiral Mike Rogers to testify on Monday as part of their committee's probe into Russia's meddling in the 2016 US presidential election.
Last year, US intelligence agencies unanimously concluded that Russia hacked and leaked Democratic Party emails during the election campaign with the intention of helping Donald Trump get elected.
But Russia has denied attempting to influence the election. Trump also cast doubt on the findings of his intelligence agencies.
Last week, new information surfaced about more than $65,000 that Michael Flynn, the former National Security Advisor, was paid in 2015 by companies with links to Russia.
Flynn resigned in February after it emerged that he lied during his confirmation hearing about his links to Russians.
TRT World's Harry Horton has this preview of what to expect from Monday's testimony.
Trump's wiretap accusations
Comey is also expected to comment on Trump's accusation that former President Barack Obama wiretapped Trump Tower.
Schiff called Trump's claims "patently false" and said he expected Comey to say as much on Monday.
Nunes said after receiving the material, he saw no evidence of wiretapping.
Nunes also said ahead of Monday's hearing he had seen "no evidence" of collusion between Russia and Trump's team.
But Schiff said there was enough "circumstantial evidence" that he still had questions.
US lawmakers from both parties said on Sunday that there is no proof on Obama's wiretapping Trump last year, adding pressure on Trump to either produce the evidence or withdraw the allegation.
The Justice Department on Friday delivered documents to Congressional committees to help clear up whether the Obama administration spied on Trump.
Also, several Republicans last week urged Trump to apologise for the allegations he made on March 4 without providing any evidence of his claims.