Rep. Adam Schiff told CNN Thursday that he had seen additional evidence, but would not specify what it was.
"We continue to get new information that, I think, paints a more complete picture of at least what we know at the outset of our investigation," Schiff said.
Asked to explain his comments earlier in the week when he said there was more than just "circumstantial evidence of collusion," Schiff said, "I do think that it's appropriate to say that it's the kind of evidence that you would submit to a grand jury at the beginning of an investigation.
"It's not the kind of evidence that you take to a trial jury when you're trying to prove something beyond a reasonable doubt. But we're at the beginning of an investigation, and given the gravity of the subject matter, I think that the evidence certainly warrants us doing a thorough investigation."
An aide said that Schiff was not suggesting that a grand jury should be empaneled, but was using an analogy drawing upon his time as a prosecutor.
On Thursday night, House Intelligence Chairman Devin Nunes said he has "no idea" and is "not familiar" with the evidence Nunes described.
The California Democrat said it was "baffling" that his Republican counterpart on the committee, Nunes, would not rule out that the information he disclosed Wednesday -- the California Republican said communications of Trump and his associates may have been collected by intelligence agencies -- came from the White House.
Nunes' disclosure -- and decision to speak to the press and White House before informing the Democrats on the Intelligence Committee -- infuriated Democrats and led him to apologize earlier Thursday, according to Rep. Jackie Speier, D-California. Other Democrats have said Nunes' actions mean he can't conduct an impartial investigation into potential Trump-Russia ties.
Schiff, however, told CNN that Democrats won't boycott the investigation, saying if they don't participate, "there isn't going to be an investigation in the House."
Nunes later told CNN he wasn't aware of Schiff's new evidence. Nunes again declined to reveal his sources for his claim -- not even to Schiff, the California Republican said.
The tension on the Intelligence Committee comes a day after CNN reported that the FBI has information that may indicate associates of Trump communicated with suspected Russian operatives to possibly coordinate the release of information damaging to Hillary Clinton's campaign, according to US officials.
The FBI is now reviewing that information, which includes human intelligence, travel, business and phone records and accounts of in-person meetings, according to those US officials. The information is raising the suspicions of FBI counterintelligence investigators that the coordination may have taken place, though officials cautioned that the information was not conclusive and that the investigation is ongoing.
White House press secretary Sean Spicer responded to the report Thursday, saying the term "associates" was a "catch-all" and overly vague.
"I have a problem with specifically -- with what your network did yesterday," Spicer told CNN's Sara Murray in a lengthy exchange at the White House press briefing. "It was associates that may or may not be there -- all one subjective term after another with no concrete proof that anything happened. When you use a term like 'associates,' you don't even put a timeframe around. It's a little bit nebulous, at best, to suggest that somebody over and over again, making the claim that you do, and the narrative continues without any substantiation."