The White House hasn't given any more details about what Trump was referring to.
Here's what we know (and what we don't) about Trump's unsubstantiated wiretapping allegations:
What exactly did Trump allege?
There are three important parts of Trump's unsubstantiated wiretapping allegations:
Claim No. 1: Trump's phones were wiretapped
What Trump is saying: Trump's tweets say he "just found out" about "the fact that President Obama was tapping my phones in October" — not adding any qualifiers. Trump hasn't offered any evidence to support his claim.
Claim No. 2: Obama ordered or was at least involved in the alleged wiretapping
What Trump is saying: In four tweets, Trump directly ties Obama to the alleged wiretapping, saying "Obama had my 'wires tapped'" and "Obama was tapping my phones." Trump hasn't offered any evidence to back those claims.
What the White House is saying: On "Good Morning America" on Monday, Sanders backed off the assertion that Obama was personally responsible, telling ABC News, "Whether it was directly ordered by this president specifically, his administration could have done this." On "This Week" on Sunday, she said "there certainly could have been" a FISA order that Obama did not order. She pointed to then–Attorney General Loretta Lynch's meeting with former President Bill Clinton on a tarmac during a Justice Department investigation into Hillary Clinton's use of a personal email server as evidence that Obama "got directly involved" in other investigations. (Lynch and Bill Clinton denied discussing the email case.) Sanders added that even if Obama wasn't involved, it "would have fallen under this administration and this past president."
What Obama is saying: A spokesperson for Obama said, "No White House official ever interfered with any independent investigation led by the Department of Justice," in a statement on Saturday. "Neither President Obama nor any White House official ever ordered surveillance on any U.S. citizen. Any suggestion otherwise is simply false." The statement did not rule out that wiretapping was initiated by the Department of Justice. Former national security adviser Ben Rhodes tweeted a similar sentiment in a response to Trump, writing, "No President can order a wiretap. Those restrictions were put in place to protect citizens from people like you."
What other officials are saying: FBI Director James Comey asked the Justice Department to publicly refute Trump's assertion that Obama ordered a wiretap of Trump's phones before the election, government sources told ABC News. Comey was concerned the president's tweets — which he believes are inaccurate — created the impression that the FBI acted improperly, and he wanted to set the record straight, the sources said. The FBI and Department of Justice declined to comment.
What we don't know: While Obama has denied any order or involvement and Comey asked his superiors for a public refutation, it's not clear whether it's possible a Justice Department investigation may have existed independently of Obama's influence or whether these top officials may be choosing not to publicly confirm the existence of such a highly classified and sensitive investigation. And while Clapper has denied the existence of a FISA Court order, we don't know whether there may have been a criminal wiretap warrant outside of a foreign intelligence-related investigation or if a state or local agency was involved.
Claim No. 3: The legality of the alleged wiretapping is questionable and 'a good lawyer could make a great case'
What Trump is saying: Trump asked via Twitter, "Is it legal for a sitting President to be 'wire tapping' a race for president prior to an election?" He added that Obama was allegedly "turned down by court earlier." He compared the alleged action to "Nixon/Watergate." Trump hasn't offered any evidence supporting his claim.
What the White House is saying: On "This Week," Sanders told Raddatz that if this wiretap happened, "this would be the greatest abuse of power and overreach that's probably ever occurred in the executive branch." And Monday on "Good Morning America," Sanders asserted to Stephanopoulos more generally that "the administration was wiretapping American citizens" and "the fact that it was being done is a fact that we should be talking about." A spokesperson for Obama said, "No president can order a wiretap." The FISA Court has approved over 10,000 requests for electronic surveillance from the Justice Department since 2009.
What other officials are saying: Former Obama senior adviser David Axelrod tweeted, "If there were the wiretap @realDonaldTrump loudly alleges, such an extraordinary warrant would only have been OKed by a court for a reason."
What the law says: A wiretap order on Trump could have been legally obtained from the FISA Court, a secret tribunal with legal authority to grant warrants for electronic surveillance against suspected spies or terrorists. The court — made up of 11 federal judges serving seven-year terms and selected by the chief justice of the United States — meets in private, sometimes in the middle of the night. FISA targets are highly classified. A FISA Court order is not the only way to authorize a wiretap: Nonintelligence, criminal wiretap warrants can be obtained but require that the individual under surveillance is using the device in connection with a past or possible future crime.
What we don't know: While Clapper has said he's not aware of any foreign-intelligence-related court order and would be aware if there was one and Obama denied ordering a wiretap of Trump, we don't know whether a criminal wiretap order remains sealed or if Trump is alleging — as with Richard Nixon and Watergate — some kind of extragovernmental group working illegally.