“He stands by it,” Spicer said, going on to assail journalists for the way they have reported on the controversy.
Earlier Thursday, the Republican and Democratic leaders of the Senate Intelligence Committee said their investigation into the matter so far has turned up no evidence that the U.S. government had conducted surveillance on Trump Tower in New York, either before or after the election on Nov. 8.
“Based on the information available to us, we see no indications that Trump Tower was the subject of surveillance by any element of the United States government either before or after Election Day 2016,” read the statement by Sen. Richard Burr (R-N.C.), the committee chairman, and Sen. Mark R. Warner (Va.), the committee's ranking Democrat.
In a Fox News Channel interview on Wednesday, Trump acknowledged that he based his initial accusation on news reports that referred to wiretapping generally. He said that information would soon be revealed that could prove him right.
“I think you're going to find some very interesting items coming to the forefront over the next two weeks,” Trump told Fox host Tucker Carlson.
Spicer refused to say whether Trump was relying on classified information to base his claims, and pointed repeatedly to news articles and “widespread, open source material.”
“The president said last night that there would be additional information coming forward,” Spicer said. “There are a ton of media reports out there that indicate that something was going on during the 2016 election.”
He added that he believed the president would ultimately be “vindicated.”
Spicer spent nearly 10 minutes reading a litany of news stories, including from conservative reporters and the New York Times, that discuss reports of inquiries into the president's campaign aides and Russian officials. But it did not appear that any of the reports confirm that Trump Tower was wiretapped.
Asked whether the president has directed the Justice Department to provide information in support of his claim to the relevant congressional committees, Spicer said that he had not.
“I think the appropriate process is to allow the House and Senate to do this so that it doesn't appear that we are interfering,” Spicer said. “We're allowing that process to play through.”
At one point, Spicer quoted a report from Fox News alleging that Obama relied on British intelligence to gain access to transcripts of conversations involving Trump. Spicer would not say if the president had raised the allegation with Prime Minister Theresa May of Britain, a close ally of the United States.
“All we're doing is literally reading off what other stations and people have reported,” Spicer said. “We're not casting judgment on that.”
In response to the allegations, which were first made by Fox News analyst Andrew Napolitano this week, the British intelligence agency GCHQ responded flatly denying any such surveillance occurred.
"Recent allegations made by media commentator Judge Andrew Napolitano about GCHQ being asked to conduct 'wire tapping' against the then President Elect are nonsense," the agency said. "They are utterly ridiculous and should be ignored."