Donald Trump's claim on Twitter that his phones were tapped by President Obama during the election is not true and "didn't happen", said the chair of a congressional committee investigating the affair.
It is the strongest assertion yet from Republican Devin Nunes, whose House Intelligence Committee is holding a public session on Monday to hear evidence on whether Russia tried to help the Trump election campaign.
"Was there a physical wiretap of Trump Tower? No there never was," Mr Nunes said
His comments follow briefings given to politicians on Friday by the Justice Department which had been asked to hand over any evidence of court orders or warrants related to Mr Trump or his family and associates.
The committee meeting on Monday will hear rare testimony from the heads of both the FBI and the secretive National Security Agency who are expected to be asked whether any investigations have been or are looking into contact between the Trump team and Russia or Russian nationals.
"We're going to highlight the fact that we know the Russians were trying to get involved in our campaign, like they have for many decades," Mr Nunes said.
He promised to try to "get to the bottom of" whether Russians tried to help the Trump campaign and "whether Trump associates or anyone else was involved".
The White House has faced a storm of criticism for Mr Trump's wiretapping claims which were made two weeks ago following damaging revelations about contacts between his officials and the Russian ambassador to the US Sergey Kislyak.
Mr Trump was accused by former CIA director Leon Panetta of making the claim as a deliberate diversionary tactic.
"They are trying to obfuscate and trying to cover up. They are trying to somehow raise other issues," he said.
Whether or not Congressional committees find evidence of surveillance of the Trump campaign or of Russian influence they are determined to track down the source of newspaper reports of undisclosed contact between former national security adviser Michael Flynn and Mr Kislyak, described by US intelligence sources as a spy recruiter.
Mr Flynn was fired by the president last month after his assurances that he had not talked to Russian officials were proved to be false.
"The one crime we know that's been committed is that one: the leaking of someone's name," said Mr Nunes.