Rep. Adam Schiff of California, the top Democrat on the committee, said it's important to determine the extent of Russia's involvements and whether they were aided by US citizens, "including people associated with the Trump campaign".
Hours before the House Intelligence Committee opened the first public hearing into both controversies, Trump sought to steer the news focus by branding the Russian Federation issue, which has been a cloud over his November presidential victory, "fake news".
Former intelligence chief "James Clapper and others stated that there is no evidence Potus colluded with Russian Federation. This story is FAKE NEWS and everyone knows it!" Trump wrote at 6:30 a.m. ET, adding, "The Democrats made up and pushed the Russian story as an excuse for running a bad campaign".
He also posted, "The real story that Congress, the Federal Bureau of Investigation and all others should be looking into is the leaking of Classified information. Must find leaker now!"
FBI Director James Comey cautioned against believing reporting on classified information, which he said oftentimes gets the facts wrong.
Democrats say they suspect members of Trump's campaign colluded with Moscow in its efforts to interfere in the November presidential election to boost Trump.
The New York Times and CNN reported in February that US intelligence officials had evidence of repeated contacts between some Trump campaign associates and Russian officials.
The claims have riveted the American public for weeks, and the stakes for the tycoon-turned-president could hardly be higher. What does matter is this: "The Russians successfully meddled in our democracy, and our intelligence agencies have concluded that they will do so again".
"And that includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia's efforts".
At the same hearing, the Republican chairman of the US House of Representatives Intelligence Committee (HIC) flatly denied that there had been a wiretap on Trump Tower in NY, but said it was possible other surveillance was used against Mr Trump.
"Let me be clear: we know there was not a wiretap on Trump Tower".
Though it's unclear exactly what the goal of the tweet is, it appears to be accusing the former president of being responsible for leaking information on the phone calls to the press, whether intentionally or unintentionally, if a staff member was behind the leaks.
Those claims sparked anger in London, where GCHQ, a close partner of U.S. spy agencies, branded the allegation "utterly ridiculous".
Manafort left the campaign in August, when news reports about his business ties to pro-Moscow Ukrainian oligarchs became a political liability.
Taken together, the disclosures in Monday's lengthy House intelligence committee hearing amounted to an extraordinary undercutting of a president, whose headline-grabbing accusations and Twitter-friendly attacks crumbled quickly under the weight of sworn congressional testimony from some of the nation's top security officials.
Trump tried to go on offense in the middle of the hearing, launching a series of tweets from his official White House account, including one that appeared to blame the Obama administration for leaking details of Flynn's contacts with the Russian envoy.
"But in unusual circumstances, where it is in the public interest, it may be appropriate to do so", Comey said. Flynn eventually resigned as President Trump's national security adviser.
Flynn later admitted talking to the ambassador about USA relations with Russia just as the Obama administration was slapping punitive new sanctions and expelling three dozen Russian officials over the election interference.
The Senate Intelligence Committee has announced a public hearing for March 30.