The chair of the House committee investigating President Trump’s accusations of wiretapping at the Trump Tower said other means of monitoring may have been used.

“Let me be clear: we know there was not a wiretap on Trump Tower, it’s still possible that other surveillance activities were used against President Trump and his associates,” Rep. Devin Nunes (R-Calif.), the chair of the House Intelligence Committee, said in a statement opening the hearing.

The ranking Democrat on the panel said it’s clear the Russians interfered in the 2016 presidential election but not whether there was collusion between Trump campaign adviser and Russian agents.

“Most important, we do not yet know whether the Russians had the help of U.S. citizens, including people associated with the Trump campaign,” said Rep. Adam Schiff (D-Calif.) “Many of Trump’s campaign personnel, including the President himself, have ties to Russia and Russian interests. This is, of course, no crime. On the other hand, if the Trump campaign, or anybody associated with it, aided or abetted the Russians, it would not only be a serious crime, it would also represent one of the most shocking betrayals of our democracy in history.”

FBI Director James Comey and National Security Agency Director Mike Rogers appeared before the committee to answer questions about Trump’s claims that former President Obama wiretapped the Trump Tower and any collusion between Trump officials and Moscow during the 2016 presidential election.

Trump, a half-hour before Comey’s appearance at the House panel, took to Twitter to blame the Democrats and to blast assertions his campaign officials cooperated with Russian agents, calling it “FAKE NEWS.”

“James Clapper and others stated that there is no evidence Potus colluded with Russia. This story is FAKE NEWS and everyone knows it!” Trump said, referring to statements made two weeks ago by the former director of national intelligence during the Obama years.

Trump added, “The Democrats made up and pushed the Russian story as an excuse for running a terrible campaign. Big advantage in Electoral College & lost!”

About two hours later, as lawmakers began to convene for the House hearing, Trump tweeted again.

“What about all of the contact with the Clinton campaign and the Russians? Also, is it true that the DNC would not let the FBI in to look?,” he wrote.

Minutes later, Schiff, the ranking Democrat on the intelligence committee, responded.

“Mr. President, the Russians hacked our election and interfered. No one disputes this now, but you. This is what is called “fact.”” he wrote.

The US intelligence community agrees that the Russians meddled in the presidential election to help Trump defeat Hillary Clinton. And the FBI has been looking into any ties between Trump advisers and Russian agents during the campaign.

Clapper, Obama’s director of national intelligence, told NBC’s “Meet the Press” on March 5, “We had no evidence of such collusion.”

On Sunday, Schiff claimed, contrary to Clapper, that there is evidence of collaboration.

“I was surprised to see Director Clapper say that because I don’t think you can make that claim categorically as he did. I would characterize it this way at the outset of the investigation: There is circumstantial evidence of collusion. There is direct evidence, I think, of deception and that’s where we begin the investigation,” Schiff said.

“There is certainly enough for us to conduct an investigation,” he added.

The House committee will hold another public hearing on March 28 with Clapper and former CIA Director John Brennan.

The Senate Intelligence Committee has announced a public hearing for March 30.

Additional reporting by Daniel Halper