(CNN)Possible collusion between Trump associates and Russians remains a subject of investigations by the House and Senate intelligence committees, contrary to assertions from President Donald Trump and White House officials that there's no evidence of collusion, multiple sources in both parties tell CNN.
Trump says no collusion; GOP and Dem members say case isn't closed
"The committee is drawing no conclusions at this time, and will continue follow the facts where they lead," GOP Sen. Richard Burr, the chairman of the Senate intelligence committee, told CNN.
Such possible collusion is "the whole underlying premise of the investigation," a House Democratic source involved in the investigation said.
Another member of the Senate committee told CNN that possible collusion "is what we are investigating."
Sen. Mark Warner, vice chairman of the Senate Select Committee on Intelligence, said while attempted Russian interference is clear, the question of whether there was collusion is still unknown.
"We know what Russia has done to interfere in the electoral process," the Virginia Democrat told CNN. "Now we have to see what kind of conversations or coordination could have taken place between individuals affiliated with the Trump campaign and the Russians. We will follow the intelligence wherever it leads."
In his termination letter to FBI Director James Comey Tuesday, Trump wrote, "I greatly appreciate you informing me, on three separate occasions, that I am not under investigation." The President has long insisted the investigation will not discover any wrongdoing.
Earlier, following Monday's Senate judiciary subcommittee hearing on Russian interference in the US election, Trump tweeted, "The Russia-Trump collusion story is a total hoax, when will this taxpayer funded charade end?"
He also tweeted, "(former Director of National Intelligence James) Clapper reiterated what everybody, including the fake media already knows- there is "no evidence" of collusion w/ Russia and Trump."
The tweet appeared to be referencing Clapper's answer to a question from Sen. Lindsay Graham, the chairman of the subcomittee, as to whether he had seen evidence of collusion. He answered that he had not.
Asked the same question by Graham, the former acting Attorney General Sally Yates, declined to answer, saying, "Senator, my answer to that question would require me to reveal classified information and so I can't answer that."
In testimony before the House intelligence committee in March, Comey testified that the FBI was investigating possible collusion between Trump associates and Russian officials and other Russians known to US intelligence as part of its broader, ongoing counter-intelligence investigation.
In addition to the FBI, four congressional panels continue to investigate Russia and Russian election interference, the Senate and House intelligence committees, the Senate Judiciary subcommittee on Crime and Terrorism, which held the hearings Monday, and the House oversight committee.
Intelligence officials tell CNN that Clapper, as a matter of policy, would not be involved in an investigation of possible collusion between Trump associates and Russia because US intelligence agencies are barred by law from probes involving US persons.
CNN has previously reported that the FBI obtained a Foreign Intelligence Surveillance Act warrant regarding Carter Page, a one-time policy adviser to then-candidate Trump, as part of its ongoing probe.