FBI Director James Comey asked the Justice Department this weekend to issue a statement refuting President Trump’s claim that President Obama ordered a wiretap of Trump’s phones before the election, but the department has not done so, according a U.S. official.

Comey made the highly unusual request on Saturday after Trump accused Obama on Twitter of having his ” ‘wires tapped’ in Trump Tower.”

The revelation, first reported by The New York Times, underscores the fraught nature of the FBI’s high-profile investigation into Russian meddling in the 2016 election. A key question fueling the probe is whether Trump associates colluded with Russian officials to help Trump win.

The department declined to comment late Sunday afternoon, as did the FBI.

The development came as Trump’s charge against Obama – leveled without any evidence – was being rebuffed both inside and outside of the executive branch. And it drew a blunt, on-the-record denial by a top intelligence official who served in the Obama administration.

Speaking on NBC News on Sunday morning, former director of national intelligence James Clapper denied that a Foreign Intelligence Surveillance Act wiretap was authorized against Trump or the campaign during his tenure.

“There was no such wiretap activity mounted against the president-elect at the time as a candidate or against his campaign,” Clapper said on “Meet the Press,” adding that he would “absolutely” have been informed if the FBI had received a FISA warrant against either.

“I can deny it,” Clapper said emphatically.

The comments followed a White House statement doubling down on Trump’s explosive series of tweets and calling for a congressional probe into “politically motivated investigations.”

In his claims early Saturday morning, the president tweeted that he “just found out” that Obama had “wires tapped” in Trump Tower before the election, comparing it to “McCarthyism.”

“Is it legal for a sitting President to be ‘wire tapping’ a race for president prior to an election?” Trump asked in another tweet. “Turned down by court earlier. A NEW LOW!”

White House press secretary Sean Spicer on Sunday cited “reports” of “potentially politically motivated investigations” during the 2016 campaign, calling them “troubling.” He did not disclose any of the reports on which the White House was basing its claim.

His statement continued: “President Donald J. Trump is requesting that as part of their investigation into Russian activity, the congressional intelligence committees exercise their oversight authority to determine whether executive branch investigative powers were abused in 2016.”

“Neither the White House nor the President will comment further until such oversight is conducted,” the statement added.

Congressional committees in the House and the Senate are probing suspected Russian efforts to undermine the 2016 election as well as any contacts between Russian officials and the Trump campaign.

Trump’s Saturday morning tweetstorm may have been prompted by the comments of a conservative radio host, which were summarized in an article on the conservative website Breitbart. The Breitbart story was circulating among Trump’s senior aides on Friday and Saturday.

A spokesman for Obama on Saturday said the former president never authorized a wiretap of Trump or any other American citizen.

The White House’s escalation of Trump’s claims were kept at arm’s length by congressional Republicans appearing on Sunday morning news broadcasts.

When asked about Trump’s allegations, Senate Intelligence Committee member Tom Cotton, R-Ark., declined to comment on the president’s tweets but said he has “seen no evidence of the allegations.”

“Whether that’s a FISA court application or denial of that application or a re-submission of that application, that doesn’t mean that none of these things happened. It just means we haven’t seen that yet,” Cotton added, speaking on Fox News Sunday.

Sen. Marco Rubio, R-Fla., said he is not aware of evidence to back up the president’s claim.

“I have no insight into exactly what he’s referring to,” Rubio said on “Meet the Press.” “The president put that out there, and now the White House will have to answer for exactly what he was referring to.”

Obama’s allies were more blunt, denying flatly that the former president had ordered a wiretap of Trump’s campaign.

“This may come as a surprise to the current occupant of the Oval Office, but the president of the United States does not have the authority to unilaterally order the wiretapping of American citizens,” said former Obama White House press secretary Josh Earnest. Senate Minority Leader Chuck Schumer, D-N.Y., told “Meet the Press” that Trump is “in trouble” and acting “beneath the dignity of the presidency.”

“The president’s in trouble if he falsely spread this kind of information,” Schumer said. “It shows this president doesn’t know how to conduct himself.”

Earnest added that Trump was attempting to distract from the controversy involving contacts between his campaign aides, including now-Attorney General Jeff Sessions, and Russian officials.

“We know exactly why president Trump tweeted what he tweeted,” Earnest added. “Because there is one page in the Trump White House crisis management playbook, and that is simply to tweet or say something outrageous to distract from a scandal. And the bigger the scandal, the more outrageous the tweet.”

Appearing on ABC’s “This Week,” White House deputy press secretary Sarah Huckabee Sanders repeatedly said that the president’s allegation was worth looking into.

“He’s asking that we get down to the bottom of this, let’s get the truth here, let’s find out,” Huckabee Sanders said. “I think the bigger story isn’t who reported it, but is it true. And I think the American people have a right to know if this happened, because if it did, again, this is the largest abuse of power that, I think, we have ever seen.”

Asked whether Trump truly believes Obama wiretapped him, Huckabee Sanders deflected.

“I would say that his tweet speaks for itself there,” she said.

Clapper’s comments referred only to whether Trump campaign officials had been wiretapped. But their conversations could also have been captured by routine surveillance of Russian diplomats or intelligence operatives.

U.S. monitoring of Russian Ambassador Sergey Kislyak, for example, caught his conversations with Trump adviser Michael Flynn during the campaign. Flynn went on to become Trump’s national security adviser, but he was forced to resign last month after admitting that he had misled other senior Trump officials about the nature of those conversations.

The FBI and the National Security Agency also have obtained intercepted communications among Russians officials in which they refer to conversations with members of the Trump team, current and former U.S. officials have said.

On the broader question of apparent Russian interference in the 2016 election, Clapper urged congressional investigators to attempt to settle the issue, which he said has become a “distraction” in the political sphere.

The intelligence community found no evidence of collusion between the Trump campaign and the Russian government – at least until the end of the Obama administration, the nation’s former top spy said Sunday.

“We had no evidence of such collusion,” Clapper said on “Meet the Press.”

He added a caveat: “This could have unfolded or become available in the time since I left government.”

Whether there was any collusion is a key question fueling a wide-ranging federal probe into Russian interference in the 2016 presidential campaign.

On Jan. 6, the U.S. spy agencies collectively released a report concluding that Russia carried out cyberhacks and other “active measures” with an intent to help Trump and harm the campaign and potential presidency of Hillary Clinton. The report, Clapper pointed out, included “no evidence” of collusion with the Trump campaign.

But the investigation by the FBI, the NSA and the CIA continues. The Senate and House intelligence committees also are conducting investigations.