US President Donald Trump has described sacked FBI director James Comey a "showboat" and "grandstander"
However, the agency's acting director has contradicted the president's account that the agency had been in turmoil before Mr Comey was fired.
In testimony before the Senate Intelligence Committee, acting FBI director Andrew McCabe promised to tell the panel of any White House meddling into the agency's probe of possible Russian interference in the 2016 election.
Mr McCabe also declined to say whether he ever heard Mr Comey tell Mr Trump that the president was not a target of the investigation into possible collusion between Russia and Mr Trump's 2016 presidential campaign.
Mr McCabe testified in place of Mr Comey, who Mr Trump abruptly dismissed as FBI director on Tuesday in an action that shocked Washington.
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Democrats have accused the Republican president of trying to foil the FBI's inquiry into Russia's actions and many have called for a special prosecutor to look into the Russia matter.
"He's a showboat. He's a grandstander. The FBI has been in turmoil," Mr Trump told NBC News in his first interview since firing Mr Comey.
"I was going to fire Comey. My decision," Mr Trump said. "I was going to fire regardless of recommendation."
Mr Trump also told NBC News he never pressured Mr Comey into dropping the FBI probe, adding, "If Russia did anything, I want to know that".
He said there was no "collusion between me and my campaign and the Russians" but added that "the Russians did not affect the vote."
The former FBI director earlier said he will "be fine," and will not dwell on how he was fired.
The anger behind Mr Trump's decision had been building for months, but a turning point came when Mr Comey refused to preview for top Trump aides his planned testimony to a Senate panel, White House officials said.
Mr Trump, Attorney General Jeff Sessions and deputy Attorney General Rod Rosenstein had wanted a heads-up from Mr Comey about what he would say at a 3 May hearing about his handling of an investigation into former Democratic presidential candidate Hillary Clinton's use of a private email server.
When Mr Comey refused, Mr Trump and his aides considered that an act of insubordination and it was one of the catalysts to Mr Trump's s decision this week to fire the FBI director, the officials said.
Mr McCabe's testimony contradicted Mr Trump's appraisal of turmoil at the FBI under Mr Comey.
"I can tell you also that Director Comey enjoyed broad support within the FBI and still does to this day," Mr McCabe testified.
"I can tell you that I hold Director Comey in the absolute highest regard. I have the highest respect for his considerable abilities and his integrity," Mr McCabe added.
And it has been the greatest privilege and honour of my professional life to work with him
Republican Committee Chairman Richard Burr asked Mr McCabe whether he ever heard Mr Comey tell Mr Trump the president was not the subject of investigation.
Mr McCabe sidestepped the question, saying he could not comment on an ongoing investigation.
In his letter firing Mr Comey on Tuesday, Mr Trump wrote, "While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the Bureau."
Mr McCabe said it was not typical practice to tell a person they are not a target of an investigation.
It is my opinion and belief that the FBI will continue to pursue this investigation vigorously and completely
Mr McCabe added that he saw no "crisis of confidence within the leadership of the FBI".
US intelligence agencies concluded in a January report that Russian President Vladimir Putin ordered an effort to disrupt the election that included hacking into Democratic Party emails and leaking them, with the aim of helping Mr Trump.
Leaders of the US intelligence agencies, including Director of National Intelligence Dan Coats and CIA chief Mike Pompeo, testifying this evening said they agreed with that finding.
Russia has denied any such interference and the Trump administration denies allegations of collusion with Russia.
Today’s hearing was scheduled to address global security threats, but there were frequent questions about Mr Comey and the FBI's Russia probe.
Several congressional panels are also investigating Russia's role in the election.
The Senate intelligence committee's top Democrat, Mark Warner, said in light of Mr Comey's firing: "It is important to restate the critical importance of protecting the independence and integrity of federal law enforcement."
"And while it's clear to me now more than ever that an independent special counsel must be appointed, make no mistake our committee will get to the bottom of what happened during the 2016 presidential election," Mr Warner said, although he said he was not happy with the pace of the committee's probe.
"The timing of this firing is wrong to anyone with a semblance of ethics," added Democratic Senator Ron Wyden.
Senate Judiciary Committee Chairman Chuck Grassley called on the FBI to publicly confirm whether or not it is probing Mr Trump.
In a statement, Mr Grassley said that Mr Comey had previously briefed him and the panel's top Democrat, Senator Dianne Feinstein, and that the politicians "heard nothing that contradicted the president's statement."
Mr McCabe said he has met with Mr Trump this week but that the Russia investigation did not come up, adding that he also has not spoken about the probe with Attorney General Jeff Sessions.
White House spokeswoman Sarah Huckabee Sanders said yesterday the Russia probe was one of the smallest things the FBI is dealing with, but Mr McCabe testified: "We consider it to be a highly significant investigation."
Mr McCabe added that Mr Comey's firing did not force a curtailment of the probe and said the agency has adequate resources to pursue it.
A congressional source said yesterday Mr Comey, days before being fired, told politicians he had sought more resources for the agency's Russia investigation.
The Trump administration has said Mr Comey's firing was unrelated to the Russia investigation.
Former Republican congressman Mike Rogers is being considered as a candidate to replace Mr Comey, a senior White House official said.
Deputy Attorney General Rod Rosenstein, who played an instrumental role in the firing, ventured to Capitol Hill to speak with members of the Senate panel.
Asked about media reports that Mr Rosenstein this week had threatened to quit, Mr Burr said that issue was not brought up.