Former CIA Director John Brennan has said an investigation into possible collusion between Trump campaign officials and the Kremlin is "well-founded".
He told the House Intelligence Committee he was aware of intelligence showing contact between Russian officials and "US persons involved in the Trump campaign".
Mr Brennan said the Russians "brazenly interfered" in last November's US elections and were "very aggressive".
But he said he did not know if the Trump campaign intrigued with Moscow.
Mr Brennan, who stepped down as CIA director in January, testified on Tuesday: "I encountered and am aware of information and intelligence that revealed contacts and interactions between Russian officials and US persons involved in the Trump campaign that I was concerned about because of known Russian efforts to suborn such individuals.
"It raised questions in my mind whether or not Russia was able to gain the co-operation of those individuals."
His evidence undercuts President Donald Trump's claim that the investigation is a "taxpayer funded charade".
The White House said Mr Brennan's testimony "backs up what we've been saying all along".
"There is still no evidence of any Russia-Trump campaign collusion," the administration said in a statement.
The House inquiry is one of two congressional investigations into claims that Russian hackers tried to tip the presidential election in Mr Trump's favour last November, and whether members of his campaign aided the alleged Kremlin conspiracy.
The FBI also has its own investigation on the issue.
Mr Brennan added that he left office with many unanswered questions about Russia's influence over the election, but that the FBI's probe "was certainly well-founded and needed to look into these issues".
Mr Brennan also told lawmakers that he had warned his Russian counterpart, FSB chief Alexander Bortnikov, during a phone call last August against meddling.
He told Mr Bortnikov any attempt to interfere would "destroy any near-term prospect" of repairing relations between Washington and Moscow.
Mr Bortnikov twice denied interfering and promised to bring up the matter with Russian President Vladimir Putin, according to Mr Brennan.
As CIA Director, Mr Brennan, along with the FBI and the Office of Director of National Intelligence, released an intelligence report in January concluding that Moscow attempted to influence the outcome of the election.
Days afterwards, Mr Trump strained relations with the intelligence community when he accused spy officials of leaking allegations that Russia had compromising information on him, likening it to "Nazi Germany".
At the time, Mr Brennan called Mr Trump's accusations "outrageous".
In a separate congressional hearing on Tuesday, Director of National Intelligence Dan Coats declined to comment on a report that Mr Trump asked him to publicly deny there was any evidence of collusion between his campaign and Moscow.
According to the Washington Post, Mr Coats and Admiral Mike Rogers, director of the National Security Agency, rejected Mr Trump's alleged request.
Meanwhile, the Senate Intelligence Committee announced plans to issue two new subpoenas - legal summons - to businesses owned by Michael Flynn, Mr Trump's fired national security adviser, who left after misleading the White House about his Russian contacts.
Also on Tuesday, Democrats on the House of Representatives Financial Services Committee wrote to the Treasury Secretary seeking "all pertinent financial records... that may shed light on President Trump's financial transactions with and business ties to Russia".
The letter, which was sent on committee letterhead, required approval from the panel's ranking Republicans in order to be sent.