A pair of United States senators this week launched a formal inquiry into evidence that supports President Donald Trump‘s claim that former President Barack Obama wiretapped Trump Tower ahead of the 2016 presidential election.

Sens. Lindsey Graham (R-S.C.) and Sheldon Whitehouse (D-R.I.) wrote in a letter to leadership of the Department of Justice and the FBI asking that they turn over any evidence they may have to support the president’s explosive but thus far unsubstantiated allegations against his predecessor.

“We request that the Department of Justice provide us copies of any warrant applications and court orders—redacted as necessary to protect intelligence sources and methods that may be compromised by disclosure, and to protect any ongoing investigations—related to wiretaps of President Trump, the Trump Campaign, or Trump Tower,” the senators wrote in their letter, which was address to Acting Deputy Attorney General Dana J. Boente and FBI Director James Comey.

“As Chairman and Ranking Member of the Senate Judiciary Committee’s Subcommittee on Crime and Terrorism, we would take any abuse of wiretapping authorities for political purposes very seriously,” the letter continues. “We would be equally alarmed to learn that a court found enough evidence of criminal activity or contact with a foreign power to legally authorize a wiretap of President Trump, the Trump Campaign, or Trump Tower.”

Graham told CNN on Wednesday that he was prepared to subpoena U.S. intelligence agencies for evidence that the Obama administration surveilled Trump.

Early Saturday morning, Trump tweeted a series of allegations against Obama, asserting without ambiguity or supporting evidence that Obama tapped his phones at Trump Tower in October. If true, it may mean Obama perpetrated the greatest illegal government scandal in a generation—or that the FBI possessed evidence that Trump or his associates acted illegally, allowing investigators to obtain a warrant.

If false, Trump would be grossly abusing his position as president to spread lies about his political opponents.

Terrible! Just found out that Obama had my "wires tapped" in Trump Tower just before the victory. Nothing found. This is McCarthyism! — Donald J. Trump (@realDonaldTrump) March 4, 2017

Is it legal for a sitting President to be "wire tapping" a race for president prior to an election? Turned down by court earlier. A NEW LOW! — Donald J. Trump (@realDonaldTrump) March 4, 2017

I'd bet a good lawyer could make a great case out of the fact that President Obama was tapping my phones in October, just prior to Election! — Donald J. Trump (@realDonaldTrump) March 4, 2017

How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy! — Donald J. Trump (@realDonaldTrump) March 4, 2017

In a statement through his spokesman, Obama flatly denied Trump’s allegations as “simply false.”

“A cardinal rule of the Obama administration was that no White House official ever interfered with any independent investigation led by the Department of Justice,” Kevin Lewis, an Obama spokesman, said in a statement Saturday. “As part of that practice, neither President Obama nor any White House official ever ordered surveillance on any U.S. citizen. Any suggestion otherwise is simply false.”

Comey has also reportedly denied that the FBI obtained a warrant to wiretap Trump tower, a denial Trump has reportedly refused to accept.

To date, the White House has provided no concrete evidence to support Trump’s claim. The president instead reportedly tweeted the allegations after reading a speculative story on Breitbart. (Trump’s chief strategist, Steve Bannon, ran Breitbart before joining Trump’s campaign.) The White House has, instead, pointed to a number of articles from a range of outlets, including the BBC and the New York Times, that cite wiretap activities surrounding the FBI’s reported investigation into Russia’s attempts to meddle with the 2016 election. No report besides Breitbart indicates that the surveillance targeted Trump Tower or the president himself.