Transcript for Trump claims 'you'll see it' when asked for proof of alleged 'spying' scandal

Next tonight, president trump escalating his war of words with the FBI tonight, unleashing a Twitter storm about his unproven claim that there might have been an FBI spy who infiltrated his campaign, in his words. He's already called the FBI's Russia investigation a witch hunt, and now this new claim tonight, he's now calling it spygate. But this evening here, a fact check. The American people already knew before the election that the FBI was investigating Russians meddling in the election, so, would FBI informants be standard procedure? Here's ABC's chief white house correspondent Jonathan Karl. Reporter: In started with a few tweets, and now, with no apparent evidence, president trump is loudly proclaiming the FBI and justice department planted spies in his 2016 campaign. We now call it spygate. Reporter: The tweets came in rapid succession, calling the alleged conspiracy "One of the biggest scandals in history," perpetrated by a, quote, "Criminal deep state." "A spy put there by the previous administration for political purposes." With no known evidence any of that happened, the president was asked today for proof. All you have to look at the basics. And you'll see. It looks like a very serious event. I hope it's not true, but it looks like it is. Reporter: I asked the president who could possibly be behind such a thing. Well, I don't want to get into it yet, but I will tell you after we look at -- Reporter: President Obama? After we look at the proof. Would he know? I would certainly hope not. But I think it's going to be pretty obvious after a while. Reporter: He then pointed to something president Obama's director of national intelligence said on "The view." If you look at clapper, he sort of admitted that they had spies in the campaign. Reporter: But that's not what clapper said. Was the FBI spying on trump's campaign? No, they were not. They were spying on -- a term I don't particularly like -- on what the Russians were doing, Reporter: The president's allegations were triggered by a "New York Times" report that an FBI informant questioned two trump campaign advisors about their contacts with Russian operatives. The use of such informants is routine. There was no mention of a spy infiltrating the trump campaign. But this is not the first time the president has talked about espionage or surveillance without evidence. He suggested there were tapes of his conversations with James Comey. The white house later said there were not. He said, without any proof, that Obama had wiretapped trump tower. Today he was asked if so-called spygate allegations are an effort to undercut the Russia investigation. No, no, we're not undercutting. What I'm doing is a service to this country. Let's get to Jon Karl, live from the white house. Jon, there is also news this evening coming in about Jared Kushner. He's now finally obtained his security clearance, we've learned. But we're also learning tonight that he recently sat down with Robert Mueller's investigators for an interview? Reporter: He did get his security clearance, finally, and significantly, David, this happened after he went in for a second interview with special counsel Robert Mueller, an interview that lasted some seven hours and included extensive questioning about his foreign contacts, both during the campaign, in the transition. This could be good news for Kushner, because apparently nothing in that interview raised red flags to prevent him from getting that security clearance. All right, Jon Karl tonight. Jon, thank you.

This transcript has been automatically generated and may not be 100% accurate.