The White House doubled down Sunday on US President Donald Trump’s contention that president Barack Obama tapped the then-candidate’s phones during the 2016 election campaign, calling for Congress to probe potential executive-branch abuses of power.

Trump relied on conservative media sources, notably Breitbart News, to make his explosive statements on Twitter about Obama, a person familiar with the situation said. In officially stepping up their concerns, the White House again produced no evidence to back up allegations of illegal activities that Obama has denied.

“Reports concerning potentially politically motivated investigations immediately ahead of the 2016 election are very troubling,” White House press secretary Sean Spicer said in a statement on Sunday.

Trump “is requesting that as part of their investigation into Russian activity, the congressional intelligence committees exercise their oversight authority to determine whether executive branch investigative powers were abused in 2016,” Spicer said.

A spokesman for Obama said Saturday that Trump’s claims were “simply false”, while lawmakers urged the president, if he had evidence of a wiretap, to make it public or at least disclose it to relevant Congressional committees.

Trump kicked off the furor with a series of four tweets early Saturday.

“Just found out that Obama had my ‘wires tapped’ in Trump Tower just before the victory,’’ Trump wrote on his personal Twitter account. “Nothing found. This is McCarthyism!’’

The president, who regularly has access to classified information and intelligence briefings, relied on a Breitbart News report for his information about the alleged wiretap, according to the person.

Breitbart, the media outlet previously run by White House chief strategist Stephen Bannon, published a story Friday outlining actions supposedly taken by the Obama administration to monitor Trump Tower in New York during the campaign.

The story, which claimed the moves were aimed at undermining Trump’s candidacy, referenced commentary by radio host Mark Levin that made similar claims.