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 colluded 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 House inquiry is one of two congressional investigation 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 "was certainly well-founded and needed to look into these issues".