The chairman of the House intelligence committee told "Fox News Sunday" that phones at President Donald Trump's campaign headquarters in midtown Manhattan were never tapped during last year's election campaign, contrary to Trump's earlier, unsubstantiated assertion.

"Was there a physical wiretap of Trump Tower? No, there never was," Rep. Devin Nunes, R-Calif., said. "The information we received Friday continues to lead us in that direction."

Nunes added: "There was no FISA warrant I am aware of to tap Trump Tower." FISA stands for the Foreign Intelligence Surveillance Act, which requires investigators to seek a warrant from a secret court to wiretap a foreign suspect.

COMEY SET TO TESTIFY ON CAPITOL HILL ABOUT RUSSIA, HACKING, WIRETAPPING

Nunes spoke as the committee prepares to begin hearings Monday into Russia's role in cybersecurity breaches at the Democratic National Committee, as well as President Donald Trump's unsubstantiated claim that his predecessor had authorized a wiretap of Trump Tower. FBI Director James Comey and Mike Rogers, the director of the National Security Agency, are slated to testify.

Trump told Fox News' "Tucker Carlson Tonight" earlier this week that he would provide evidence of his wiretapping claim to the committee "very soon."

Nunes said the committee will also examine whether the Russians were trying to sow doubt in the U.S. electoral system or whether they were trying to help Trump get elected to the White House.

"We need to get to the bottom of that," Nunes said.

Meanwhile, ranking member Adam Schiff, D-Calif., told NBC's "Meet The Press" that documents given to the committee by the FBI and the Justice Department late last week offered circumstantial evidence that American citizens colluded with Russians in Moscow's efforts to interfere in the presidential election.

"There was circumstantial evidence of collusion; there is direct evidence, I think, of deception," Rep. Adam Schiff, D-Calif., said. "There's certainly enough for us to conduct an investigation."

Intelligence officials have said that Russia was behind the theft of Democratic National Committee emails last summer. The U.S. government later concluded that the Russian government directed the DNC hack in an attempt to influence the outcome of November's presidential election.

SENATORS: 'NO INDICATION' TRUMP TOWER WAS SUBJECT OF SURVEILLANCE

"For the first time the American people, and all the political parties now, are paying attention to the threat that Russia poses," Nunes said. "We know that the Russians were trying to get involved in our campaign, like they have for many decades. They're also trying to get involved in campaigns around the globe and over in Europe."

The Associated Press contributed to this report.