A US Republican senator has warned he will use a court order to force the FBI to say whether Barack Obama wiretapped President Donald Trump's phones.
Senator Lindsey Graham said Congress would "flex its muscle" if FBI director James Comey did not meet its requests.
The president has made unsubstantiated claims that his predecessor tapped his phones during the 2016 election campaign.
Mr Graham also said he wanted the FBI to confirm if there was a Trump-Russia probe.
The senator, who is leading the Senate Judiciary Committee's investigation of allegations of Trump-Russia ties, has said he would issue a subpoena to force Mr Comey to submit details on this and on the alleged wiretapping.
And he threatened to hold up Mr Trump's nominee for deputy attorney general, Rod J Rosenstein, until "Congress is provided with the information to finally clear the air as to whether there was ever a [wiretapping] warrant issued against the Trump campaign".
The head of the House intelligence committee, Devin Nunes, said on Wednesday he does not believe "there was an actual tap of Trump Tower".
Despite repeated requests, the White House has not given any evidence for the claim, but asked Congress to examine the allegation as part of an investigation into alleged Russian meddling in last year's election.
A spokesman for Mr Obama has said the accusation is "simply false".
No evidence of wiretapping, according to:
- former President Barack Obama
- ex-Director of National Intelligence James Clapper
- ex-CIA Director John Brennan
- Republican chairman of House intelligence committee, Devin Nunes
- Republican Senator John McCain
White House Press Secretary Sean Spicer backed off Mr Trump's initial claims, saying the president had broadly meant "surveillance and other activities", and was not accusing Mr Obama personally.
However, the president was very specific in his accusation two weeks ago in a series of tweets, asking: "Is it legal for a sitting President to be 'wire tapping' a race for president?"
Mr Graham also said he wants details about whether President Trump's campaign had any Russian ties during the presidential campaign.
The South Carolina senator and Democratic Senator Sheldon Whitehouse, both members of the Judiciary Committee, asked Mr Comey to provide information on any alleged Russian connections to the Trump campaign, before 15 March.
"He hasn't answered that letter or confirmed if there's a real investigation of the Trump campaign," Mr Graham said.
"He needs to answer the letter and give the nation some information about what's going on here."
US intelligence agencies found that Russia conducted cyber-attacks against the Democratic Party as part of an effort to influence the election in Mr Trump's favour.
Russia has repeatedly denied any involvement.
Mr Trump has been dogged by claims that his advisers and staff had ties to Russian officials, but there has been no evidence of any collusion between his campaign and Moscow.