Rep. Devin Nunes (R-Calif.) said Tuesday that he’ll never reveal the source behind his claim that members of Donald Trump’s team had indirectly come under surveillance by U.S. intelligence agencies.

“We will never reveal sources and methods,” Nunes, chairman of the House Intelligence Committee, said to ABC News’ Mary Bruce on Tuesday.

Even other members of the committee, which is investigating Russian interference in the presidential campaign, will not learn how Nunes obtained information he said indicated that key figures close to Trump were monitored by U.S. intelligence, Nunes said.

House Intel chair @DevinNunes tells @marykbruce he will never share source behind surveillance claim, even to other members of his committee pic.twitter.com/DWb3x9ADAO — ABC News Politics (@ABCPolitics) March 28, 2017

Other members of the intelligence committee have not yet seen the materials Nunes cited, and it’s unclear when they’ll be made available to other lawmakers. “My guess is that it’s going to be a slow process,” Nunes said to ABC News.

Nunes has come under fire from Democrats over his ability to lead a credible investigation into the Russian connections to the presidential campaign.

Tuesday he canceled this week’s scheduled meetings of the intelligence committee as pressure mounted for him to step aside. Nunes said that he will not recuse himself. Currently, he has the support of Speaker Paul Ryan (R-Wis.), who has the power to replace him as committee chair, The New York Times said.

Of particular concern to critics is that Nunes briefed Trump on Wednesday about evidence he said he received from an unnamed source Tuesday on the White House grounds. The evidence allegedly shows that people close to Trump came under so-called “incidental” surveillance while the spy agencies carried out monitoring of foreign targets.

Trump, in tweets March 4, had claimed without evidence that former President Barack Obama wiretapped his phones in Trump Tower in New York before the November election. Trump had said initially that he’d been “somewhat” vindicated by Nunes’ statements Wednesday about surveillance.