The leaders of a congressional inquiry into Russia's alleged interference in the US election have asked the FBI and Justice Department for any information they have on Donald Trump's unsubstantiated claim he was wiretapped by Barack Obama.
Republican Lindsey Graham and Democrat Sheldon Whitehouse declared Congress "must get to the bottom" of Mr Trump's claim.
In their letter, the senators asked FBI Director James Comey and Acting Deputy Attorney General Dana Boente to provide "copies of any warrant applications and court orders... related to wiretaps of President Trump, the Trump campaign, or Trump Tower."
Mr Graham and Mr Whitehouse said they would "take any abuse of wiretapping authorities for political purposes very seriously."
But, they added: "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."
Last weekend, Mr Trump tweeted to accuse his predecessor of tapping his phones at Trump Tower during the election, offering no evidence to back up his claim.
A spokesman for Mr Obama said neither her nor any White House official had ever ordered surveillance on a US citizen, while his director of national intelligence James Clapper said nothing matching Mr Trump's claims had taken place.
Under US law, presidents cannot direct wiretapping. Instead, the federal government can ask a court to authorise the action, but must provide justification.
When asked at a briefing if Mr Trump was the subject of a probe, White House spokesman Sean Spicer replied: "There is no reason that we have to think that the president is the target of any investigation whatsoever."
Critics of Mr 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 his long-held contention Mr 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.
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