Nunes hurried to the White House to personally brief Trump on the revelations, after talking to the press but without sharing the information with Democrats. His Democratic counterpart on the committee -- Rep. Adam Schiff of California -- warned that his colleague had cast a "profound cloud" over their effort to investigate Russian attempts to interfere in the election.
A Republican source with knowledge of the situation claimed the information that Nunes talked about was from the intelligence community and not the White House. The source said Nunes was "steaming" about what he read.
That source said Nunes met with Republican members of the Intelligence Committee before his news conference, and several tried to convince him not to do it before he spoke with Schiff. But Nunes didn't take the advice, with the news conference already called by the time he met with the GOP committee members. Nunes was too mad, the source said.
The comments by Nunes do not appear to support Trump's claims, debunked by FBI Director James Comey
, that he was wiretapped by his predecessor President Barack Obama. Rather, they appear to relate to conversations between Trump or associates and people who were targeted by FISA warrants -- that Nunes said did not appear to be illegal.
But the President was asked by reporters if he feels vindicated by the visit from Nunes and answered: "I somewhat do. I must tell you I somewhat do, I very much appreciated the fact that they found what they found."
Another day of shocking drama intensified the intrigue and uncertainty sparked by Trump's feud with intelligence agencies, his accusations of shady practices by his predecessor and the wider question of the Trump campaign's links with Russia that are currently the focus of an investigation by the FBI.
Nunes told reporters the conversations were "incidentally" collected as part of intelligence sweeps focusing on other people and implied that Trump was not the target of the surveillance operation.
He said he discovered accounts of conversations related to Trump and his associates when he was reviewing intelligence reports brought to him by an unidentified person -- and said the information was not related to Russia.
"This is a normal, incidental collection, based on what I could collect," Nunes said. "This appears to be all legally collected foreign intelligence under" the Foreign Intelligence Surveillance Act.
Nevertheless, Nunes said he alerted House Speaker Paul Ryan about the collection before he headed to the White House, adding: "I'm actually alarmed by it." Democrats on the committee said they were not informed of Nunes' finding before he spoke with the media.
Generally, American citizens who are caught up in surveillance of foreign targets are not identified by name in intelligence reports. But Nunes' announcement raises the question of whether the President-elect or associates were identified in intelligence reports circulating in the covert community.
A House member on the Intelligence Committee told CNN the communications in question were senior-level people talking about Trump, not Trump himself.
Nunes later told CNN's "The Lead with Jake Tapper" that "President-elect Trump and his team were put into intelligence reports."
"Clearly there is a lot of information in the reports that I've seen, which were dozens, that would lead me to believe that the last administration and numerous agencies had a pretty good idea of what President-elect Trump was up to and what his transition team was up to and who they were meeting with," Nunes told Tapper.
Democrats in the dark
In a news conference, Schiff expressed deep concern that committee members were not told by Nunes about the revelations before he briefed the press or went to the White House.
"If the chairman is going to continue to go to the White House instead of his own committee, there is no way we can continue to conduct this investigation," he said.
"It does underscore the importance of establishing an independent commission," Schiff said.
Schiff said that he and other members of the committee had still not seen the documents that Nunes was referring to. He also said from a conversation with Nunes that there did not appear to have been any "unmasking" of any of the US people mentioned in the intercepts.
"All of us are in the dark."
Nunes defended rushing to brief the President because the reports he read have nothing to do with Russia, but he said the investigation he is conducting into Russian interference in the election would look at how Trump's name got into the intelligence reports.
"Because what I saw has nothing to do with Russia and has nothing to do with the Russian investigations, (and) has everything to do with possible surveillance activities ... the President needs to know these intelligence reports are out there and I have a duty to tell him that," Nunes said.
But in an interview with CNN's Kate Bolduan on "Erin Burnett OutFront," Democratic Rep. Jackie Speier slammed Nunes as a "presidential whisperer" undermining the committee's investigation and accused the White House of planning the day's events.
"This was pure theater," Speier said. "I think in many respects it was probably orchestrated by the White House."
Still no sign of wiretapping
Two weeks ago, Trump asked Congress to investigate whether Trump Tower
was wiretapped by his predecessor.
On Monday, Nunes' committee held a hearing featuring Comey and National Security Agency Director Mike Rogers. At that hearing, Comey confirmed for the first time that his agency
is investigating possible collusion between Russia and the Trump campaign and whether any crimes may have been committed during last year's election campaign as part of a wider probe into the hacking of Democratic servers and the Clinton campaign.
Also at that hearing, Comey said he had seen no evidence so far of the specific allegation of wiretapping Trump Tower.
Nunes at his news conference said he did not know whether the "incidental collection" happened at Trump Tower, and could not say for certain whether Trump's communications were directly collected. He said the collection included Trump transition officials and that it happened after the election.
UPDATED CORRECTION: A previous version of this story misstated whom Nunes met with before his news conference. He met Republican members of the House Intelligence Committee.