WASHINGTON (AP) — Piece by piece, acting FBI Director Andrew McCabe undermined recent White House explanations about the firing of FBI Director James Comey during testimony before a Senate committee Thursday.

Since President Donald Trump's surprise ouster of Comey on Tuesday, the White House has justified his decision, in part, by saying that the director had lost the confidence of the rank and file of the FBI as well as the public in general.

"That is not accurate," McCabe said in a response to a senator's question about the White House assertions. "I can tell you also that Director Comey enjoyed broad support within the FBI and still does to this day."

The firing of Comey left the fate of the FBI's probe into Russia's election meddling and possible ties to the Trump campaign deeply uncertain. The investigation has shadowed Trump from the outset of his presidency, though he's denied any ties to Russia or knowledge of any campaign coordination with Moscow.

McCabe called the investigation "highly significant" — another contradiction of the White House portrayal — and assured senators Comey's firing will not hinder it. He promised senators he would tolerate no interference from the White House and would not provide the administration with updates on its progress.

"You cannot stop the men and women of the FBI from doing the right thing," he declared. He said there has been no interference so far.

Trump, in a letter to Comey dated Tuesday, contended that the director had told him "three times" that he was not personally under investigation. McCabe said it is not standard FBI practice to tell someone he or she is or isn't under investigation.

The White House refused Wednesday to provide any evidence or greater detail. Former FBI agents said such a statement by the director would be all but unthinkable. McCabe told senators he could not comment on conversations between Comey and the president.

Days before he was fired, Comey requested more resources to pursue his investigation, U.S. officials have said, fueling concerns that Trump was trying to undermine a probe that could threaten his presidency. McCabe said he was not aware of any such request and said the Russia investigation is adequately resourced.

It was unclear whether word of the Comey request, put to Deputy Attorney General Rod Rosenstein, ever made its way to Trump. But the revelation intensified the pressure on the White House from both political parties to explain the motives behind Comey's stunning ouster.

Trump is the first president since Richard Nixon to fire a law enforcement official overseeing an investigation with ties to the White House. Democrats quickly accused Trump of using Comey's handling of the Hillary Clinton email investigation as a pretext and called for a special prosecutor into the Russia probe. Republican leaders brushed off the idea as unnecessary.

Defending the firing, White House officials said Trump's confidence in Comey had been eroding for months. They suggested Trump was persuaded to take the step by Justice Department officials and a scathing memo, written by Rosenstein, criticizing the director's role in the Clinton investigation.

"Frankly, he'd been considering letting Director Comey go since the day he was elected," White House spokeswoman Sarah Huckabee Sanders said, a sharply different explanation from the day before, when officials put the emphasis on new Justice complaints about Comey.

Sanders, speaking Thursday on ABC's "Good Morning America," said, "I have heard that directly from him (Trump), that that information was relayed directly to him from Director Comey." On NBC's "Today," Sanders said she would defer to the president himself for any additional details.

Outraged Democrats called for an independent investigation into the Trump campaign's possible ties to Russia's election interference, and a handful of prominent Republican senators left open that possibility. But Senate Majority Leader Mitch McConnell, with the support of the White House, brushed aside those calls, saying a new investigation would only "impede the current work being done."

The Senate intelligence committee on Wednesday subpoenaed former Trump national security adviser Michael Flynn for documents related to its investigation into Russia's election meddling. Flynn's Russia ties are also being scrutinized by the FBI.

The White House appeared caught off guard by the intense response to Comey's firing, given that the FBI director had become a pariah among Democrats for his role in the Clinton investigation. In defending the decision, officials leaned heavily on a memo from Rosenstein, the deputy attorney general, criticizing Comey's handling of the Clinton investigation.

But Rosenstein's own role in Comey's firing became increasingly murky Wednesday.

Three U.S. officials said Comey recently asked Rosenstein for more manpower to help with the Russia investigation. Sen. Dick Durbin, D-Ill., said that while he couldn't be certain the request triggered Comey's dismissal, he said he believed the FBI "was breathing down the neck of the Trump campaign and their operatives and this was an effort to slow down the investigation."

Justice Department spokeswoman Sarah Isgur Flores denied that Comey had asked Rosenstein for more resources for the Russia investigation.

Trump advisers said the president met with Rosenstein, as well as Attorney General Jeff Sessions, on Monday after learning that they were at the White House for other meetings. One official said Trump asked Rosenstein and Sessions for their views on Comey, then asked the deputy attorney general to synthesize his thoughts in a memo.

The president fired Comey the following day. The White House informed Comey by sending him an email with several documents, including Rosenstein's memo.

It's unclear whether Rosenstein was aware his report would be used to justify the director's ouster.

White House and other U.S. officials insisted on anonymity to disclose private conversations.

A farewell letter from Comey that circulated among friends and colleagues said he does not plan to dwell on the decision to fire him or on "the way it was executed."

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AP writers Darlene Superville, Ken Thomas, Vivian Salama, Catherine Lucey and Eric Tucker in Washington contributed to this report.