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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