Two senior senators have asked the FBI and Justice Department for any information they have on President Donald Trump's unsubstantiated claim that his predecessor Barack Obama wiretapped him during the 2016 US presidential campaign.
In a letter to James Comey, director of the Federal Bureau of Investigation, and Acting Deputy Attorney General Dana Boente, Republican Senator Lindsey Graham and Democratic Senator Sheldon Whitehouse wrote:
"We request that the Department of Justice provide us copies of any warrant applications and court orders...related to wiretaps of President Trump, the Trump campaign, or Trump Tower."
Under US law, presidents cannot direct wiretapping. Instead, the federal government can ask a court to authorise the action, but it must provide justification.
Critics of Trump in Congress have accused him of issuing the wiretap allegation to try to deflect attention from investigations into his administration's possible ties to Russia.
Some have likened it to Trump's long-held contention that Obama was not born in the United States and thus did not legitimately hold the office of president - an accusation he did not withdraw until 2016.
Graham later told CNN that if the Justice Department does not co-operate with the senators' request, subpoenas would be issued.
"I expect them to come forward as to whether or not a warrant was obtained or sought," Graham said.
On Saturday, Trump accused Obama of phone surveillance, amid a swirl of questions about possible ties between the Trump presidential campaign and Russian President Vladimir Putin.
An Obama spokesman said on Saturday that neither Obama nor any White House official had ordered surveillance on any US citizen.
The spokesman's statement did not address the possibility that a wiretap of the Trump campaign could have been sought by the Justice Department.
In their letter, Graham and Whitehouse wrote: "We would take any abuse of wiretapping authorities for political purposes very seriously."
"We would be equally alarmed to learn that a court found enough evidence of criminal activity or contact with a foreign power to legally authorise a wiretap of President Trump, the Trump campaign, or Trump Tower."
On Tuesday, House Intelligence Committee Chairman Devin Nunes told reporters his panel would consider issuing subpoenas if information being sought on Russia's possible role influencing the election was not forthcoming.
