U.S. Federal Bureau of Investigation Director James Comey asked the Justice Department to dispute President Donald Trump's allegation that former President Barack Obama ordered a wiretap on telephones at Trump Tower in New York last year.
U.S. officials who spoke to the Associated Press, Washington Post and New York Times said Comey's request followed Trump's accusation on Twitter Saturday that included a comparison to former President Richard Nixon, who resigned amid scandal in 1974. Trump has offered no evidence to support his claim.
What is not clear is why Comey did not dispute the statement himself. As FBI director under Obama, his department has been a lead in the ongoing investigation of Russian influence on last year's election.
Under U.S. law, a president cannot order someone's phone to be wiretapped. Such a move would require approval by a federal judge and be based on reasonable grounds to suspect why a citizen's telephone calls should be monitored.
Obama's Director of National Intelligence James Clapper called Trump's charge simply false.
"There was no such wiretap activity mounted against the president-elect at the time, or as a candidate or against his campaign," Clapper told NBC's Meet the Press.
WATCH: Former Director of National Intelligence James Clapper on wire tap allegations
In addition to the FBI probe, the House and Senate intelligence committees are carrying out their own investigations, including looking into what cyber activities Russia directed at the U.S. and whether those efforts had links between Russian officials and people connected to U.S. political campaigns.
House Intelligence Committee Chairman Devin Nunes said in a statement Sunday his committee "will make inquiries into whether the government was conducting surveillance activities on any political party's campaign officials or surrogates."
The top Democrat on the committee, Rep. Adam Schiff, said Trump's accusation was based on "conspiracy-based news."
"For a president of the United States to make such an incendiary charge - and one that discredits our democracy in the eyes of the world - is as destructive as it was baseless," Schiff said.
White House Press Secretary Sean Spicer issued a statement Sunday saying the president is requesting the committees to "determine whether executive branch investigative powers were abused in 2016." He added that neither Trump nor the White House would offer further comment "until such oversight is conducted."
House Democratic Leader Nancy Pelosi said Trump has no proof and is trying to shift conversation from Russia to Obama.
"This is called the wrap-up smear," she told CNN. "You make up something, then you have the press write about it and then you say everybody's writing about this charge. It's a tool of an authoritarian to just have you always be talking about what you want them to be talking about."
Senator Marco Rubio told NBC that Trump "will have to answer as to what exactly" he was referring to in making the claim that his phones were tapped.
WATCH: White House Demands Probe of Alleged Trump Tower Wiretap
Alleged Russia connection hard to shake
The publisher of the Newsmax Media website, Christopher Ruddy - a friend of Trump's - wrote Sunday the president told him, "This will be investigated. It will all come out. I will be proven right."
Ruddy said he has never seen Trump this angry in a long time.
A U.S. intelligence report concluded Russia carried out a campaign at the direction of President Vladimir Putin that used cyber attacks and other methods to influence the U.S. election campaign with the aspiration of helping Trump's chances of beating Democratic challenger Hillary Clinton.
Trump has denied any links to Russia.
His first national security advisor, Michael Flynn, resigned last month after information emerged that he had lied to top officials about the nature of his own conversations with the Russian ambassador.
Last week, Attorney General Jeff Sessions decided to remove himself from any investigation into the Russian activities after reports emerged that he met twice last year with the ambassador, yet said at his confirmation hearing in January that he "did not have communications with the Russians."