Updated at 5:06 p.m. ET

Former FBI Director James Comey told the Senate Intelligence Committee that he believed he was fired by President Trump over the growing Russia investigation and that other arguments by the White House were "lies, plain and simple."

In bombshell testimony Thursday, Comey, who was abruptly let go a month ago, said he began documenting his numerous, and often uncomfortable, conversations with Trump — in which the president asked for his "loyalty" and for him to scuttle the FBI's investigation into former Trump national security adviser Michael Flynn — because he "was honestly concerned (Trump) might lie" about their meetings.

"I knew there might come a day when I might need a record of what happened not just to defend myself" but also the FBI, Comey added.

"My impression is something big is about to happen. I need to remember every word that is spoken," he said of the memos he wrote immediately after his encounters with the president.

White House deputy press secretary Sarah Huckabee Sanders later pushed back on Comey's assertions, telling reporters that "I can definitely say the president is not a liar and I think it's frankly insulting" that the question was asked.

"Shifting explanations" for Comey's firing

The former FBI director also zeroed in on Trump's evolving explanations for why he had been let go, saying he was "confused" and "increasingly concerned" about the "shifting explanations" Trump gave. The president initially pointed to Comey's handling of the Hillary Clinton email investigation as the impetus but later conceded the firing was because of his handling of the Russia investigation and claimed Comey was overseeing a demoralized FBI in disarray.

"So it confused me when I saw on television the president saying he actually fired me because of the Russia investigation and learned again from the media that he was telling, privately, other parties that my firing had relieved 'great pressure' on the Russia investigation," Comey said, referring to reporting on Trump's conversation with Russian officials in the Oval Office the day after the dismissal.

"The administration then chose to defame me and, more importantly, the FBI," Comey said, by claiming the agency was "poorly led."

"Those were lies, plain and simple," Comey bluntly told the committee.

Awkward "loyalty" dinners and Oval Office pressure

Comey's highly anticipated testimony followed the release by the committee of his written statement on Wednesday, which ticked off in rich detail the extent to which Trump pressed him about the Russia investigation. Comey wrote that Trump asked him for a "loyalty" pledge during a one-on-one dinner and later told Comey in a solo Oval Office meeting that he "hope[d] you can let ... go" of the Flynn investigation.

The president has denied both those accusations. But Comey's testimony only confirmed the many bombshell reports over the past few weeks about the private conversations he had with Trump, many of them unprecedented and possibly inappropriate.

Comey told the committee that he did see Trump's request for loyalty from him — an independent arbiter atop the FBI — as a sort of quid pro quo.

"My common sense told me what's going on here is he's looking to get something in exchange for granting my request to stay in the job," Comey said.

He also said that in a now-infamous photo just days after Trump's inauguration in which the president shook his hand and embraced him, Trump whispered in his ear, "I really look forward to working with you."

On Trump's Feb. 14 private conversation with Comey in the Oval Office — the day after Flynn was asked to resign for misleading Vice President Pence over his conversations with the Russian ambassador during the transition — Comey said that he believed the president was indeed directing him to scuttle the investigation into Flynn.

"I took it as a direction," which he didn't follow, Comey said. "I took it as, this is what he wants me to do." Comey also added that, at that time, Flynn was indeed in "legal jeopardy" over his ties and contacts with the Russians.

He noted that Attorney General Jeff Sessions tried to linger in on that meeting, and he said that it was his "sense" that Sessions "knew he shouldn't be leaving." According to Comey's written testimony, he later told Sessions he didn't want to be left alone with Trump again. Comey said during questioning that he was "stunned" by that conversation and the president asking everyone else to leave and later said it was a "significant fact" to him as a prosecutor that Trump wanted to speak with him alone.

Obstruction of justice? Up to special counsel Mueller to "sort that out"

Comey did testify that he didn't believe Trump was asking Comey to stop the broader Russia investigation being conducted by the FBI and that while his conversation with Trump was "disturbing," he didn't "think it's for me to say that the conversation I had with the president was an effort to obstruct."

Later asked by Sen. Joe Manchin, D-W.Va., whether Trump's interactions with him over Flynn rose to the level of obstruction of justice, Comey responded that was newly appointed special counsel Robert Mueller's "job to sort that out."

Comey said he had seen Trump's tweet last month suggesting that the president had "tapes" of their conversations — and that if those did exist, they would corroborate his testimony.

"Lordy, I hope there are tapes," he told senators.

And, Comey revealed, it was Trump's tweet about those alleged tapes that prompted him to call up a friend to leak the memos about his conversations with the president to the press last month in hopes that doing so would lead to the appointment of a special counsel to oversee the Russia investigation.

Republicans have already seized on one section of Comey's written testimony as vindication. The former FBI director does detail in his written submission to the committee how he told Trump on three occasions that the president was not himself under investigation — a surprising assertion that Trump had put in his termination letter to Comey.

Comey told the committee that deliberations with senior FBI leadership about whether to tell the president he was not personally under investigation beginning in January, before he was inaugurated, were not unanimous.

"One of the members of the leadership team had a view that although it was technically true, we did not have a counterintelligence file case open on then-President-elect Trump, his concern was that because we're looking at the potential — again that's the subject of the investigation — coordination between the campaign and Russia, because it was President Trump — President-elect Trump's campaign, this person's view was inevitably his behavior, his conduct will fall within the scope of that work. And so he was reluctant to make the statement that I made. I disagreed," Comey said. "I thought it was fair to say what was literally true — there is not a counterintelligence investigation of Mr. Trump — and I decided in the moment to say it given the nature of our conversation."

He later confirmed that when he was let go on May 9, there was no counterintelligence nor criminal investigation of Trump individually and that the president was not personally under investigation.

Asked by Sen. Tom Cotton, R-Ark., as to whether he believed Trump colluded with Russia during the campaign, Comey responded, "I don't think I should answer in an open setting." Following his open hearing with members of the intelligence committee, Comey went into a closed briefing with senators.

Comey did underscore that while there was plenty of evidence that Russians meddled in the 2016 election, he was confident that no votes were altered.

However, he also said Trump never asked him what the FBI was doing to investigate the Russian meddling or to stop it from happening again in future elections. Comey said he fully expected Russian mischief in U.S. elections to resume in subsequent electoral cycles.

Comey was also pressed by Republicans on the committee, including Florida Sen. Marco Rubio, about why he didn't alert White House officials or the White House Counsel's Office about what he believed were inappropriate conversations with Trump and a breach of protocol.

"I don't know," Comey responded to Rubio. "I think the circumstances were such that I was a bit stunned and didn't have the presence of mind."

Comey critical of Obama Attorney General Loretta Lynch

Republicans also pressed Comey on how he handled the investigation into Hillary Clinton's emails during the 2016 campaign, usurping then-Attorney General Loretta Lynch by announcing the FBI was not recommending prosecution for the handling of classified information on Clinton's private server.

Comey testified he was disturbed by Lynch's tarmac meeting with Clinton's husband, former President Bill Clinton, and that was one reason he made his announcement in July, though he did say at that time he believed Hillary Clinton had been careless with classified information.

"I didn't believe she could credibly decline that investigation, at least not without grievous damage to the Department of Justice and the FBI," Comey said of President Barack Obama's attorney general.

Comey said it made him uncomfortable that Lynch had directed him to refer to the Clinton email investigation as a "matter" and not an investigation, even though she was, indeed, under criminal investigation.

Pressed by Sen. John McCain, R-Ariz., as to why he didn't call for an independent counsel to handle that matter, Comey said that he "knew there was no case there" and that "calling for a special counsel would be brutally unfair."

Republicans push back that Trump has been "vindicated"

The president did not immediately respond directly to Comey's testimony and has not tweeted, as many people speculated he might. However, the president's son Donald Trump Jr. did largely live-tweet the event.

And Marc Kasowitz, President Trump's personal lawyer regarding the Russia inquiries, underscored in a statement that Comey did confirm what Trump has long asserted — that the former FBI director assured the president he was not under investigation.

"[Comey] also admitted that there is no evidence that a single vote changed as a result of any Russian interference. Mr. Comey's testimony also makes clear that the President never sought to impede the investigation into attempted Russian interference in the 2016 election, and in fact, according to Mr. Comey, the President told Mr. Comey 'it would be good to find out' in that investigation if there were 'some "satellite" associates of his who did something wrong.' And he did not exclude anyone from that statement," Kasowitz said.

Kasowitz also claimed that Trump never told Comey he needed loyalty from him "in any form or substance," even though the president "is entitled to expect loyalty from those who are serving in an administration."

And he raised alarm over Comey's admission that he had encouraged a friend to leak his memos about his conversations with the president to the press, suggesting those were privileged communications.

"We will leave it the appropriate authorities to determine whether this leaks should be investigated along with all those others being investigated," Kasowitz said.

Meanwhile, House Speaker Paul Ryan defended Trump at his weekly press conference, arguing that one reason the president had unusual conversations with Comey was because he was a novice president learning the ropes.

"Of course there needs to be a degree of independence between DOJ, FBI and the White House and a line of communications established. The president is new at this. He's new to government. So he probably wasn't steeped in the long-running protocols that establish the relationships between DOJ, FBI and White Houses," Ryan said. "He's just new to this."

Ryan was pushed by a reporter who asked how being new is an acceptable excuse when he has staff and legal counsel.

"I'm not saying it's an acceptable excuse, it's just my observation," the Wisconsin Republican replied.

"I think people now realize why the president is so frustrated," Ryan added. "When the FBI director tells him on three different occasions he's not under investigation yet the speculation swirls around the political system that he is, that's frustrating. Of course the president is frustrated, and I think the American people now know why he was so frustrated."

NPR's Susan Davis contributed to this report.

Correction: June 8, 2017 12:00 am — A previous version of this story incorrectly spelled Robert Mueller's last name as Muller.

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