Earlier this month, without offering evidence, the Republican president accused his Democratic predecessor of wiretapping him, a charge that Obama spokesman Kevin Lewis said was “simply false.”

In response, the leaders of the House Intelligence Committee sent a letter to the Justice Department requesting proof for the allegation by Monday.

A department spokeswoman said it needed more time “to review the request in compliance with the governing legal authorities and to determine what if any responsive documents may exist.”

The committee replied in a statement that it wanted a response by the time of a planned hearing on March 20, suggesting it would use a subpoena if that did not happen.

“If the committee does not receive a response by then, the committee will ask for this information during the March 20 hearing and may resort to a compulsory process if our questions continue to go unanswered,” a spokesman said.

The Justice Department is not required to respond to the representatives’ request for evidence or meet its deadline.

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Trump declined to comment on Monday when asked by reporters about the wiretapping issue.

White House spokesman Sean Spicer said Trump’s original statement, which he released on Twitter, referred to other types of surveillance besides wiretapping.

“The president was very clear in his tweet that it was wiretapping – that spans a whole host of surveillance types of options. The House and the Senate intelligence committees will now look into that and provide a report back,” he said.

In fact, Trump only referred to wiretapping in his tweet, which read: “How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!”

Under U.S. law, presidents cannot direct wiretapping. Instead, the federal government can ask a court to authorize the action, but it must provide justification.

Republicans have distanced themselves from the president over the issue. Republican Senator John McCain on Sunday called on Trump to provide evidence for his allegation or retract it.

Representative Adam Schiff, the top Democrat on the committee, has said he plans to question FBI Director James Comey over Trump’s wiretapping charge.

Comey has called on the Justice Department to deny the allegation, according to law enforcement sources.

The issue may also come up at hearing set for Wednesday by a Senate Judiciary Committee subcommittee on the methods used by Russia and other authoritarian governments “for undermining democracies throughout the world.”

The chairman of the Subcommittee on Crime and Terrorism, Republican Lindsey Graham, and the senior Democrat, Sheldon Whitehouse, wrote to Comey and acting Attorney General Dana Boente last Wednesday asking for “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.”