Acting Director of National Intelligence Joseph Maguire, in tense testimony Thursday on Capitol Hill, defended his handling of the explosive whistleblower complaint alleging President Trump pressured the Ukrainian president to investigate the Biden family -- while calling the matter "unprecedented."

But after maintaining his composure for most of the morning, his frustration showed through at the end, when House Intelligence Committee Chairman Adam Schiff, D-Calif., repeatedly pressed him to agree that the matter should be investigated.

READ THE WHISTLEBLOWER'S COMPLAINT

Maguire stressed that the committee now has all of the relevant information, including the whistleblower complaint itself that was released publicly Thursday morning, and said it's up to them to decide how to proceed.

“The horse has left the barn,” Maguire said. “You have all of the information. You have the whistleblower complaint. You have the letter from the ICIG. You have the Office of Legal Counsel opinion and you have the transcript from the president.”

Maguire also stressed that the complaint was essentially "hearsay" and not “corroborated by other folks.”

“This is second-hand information. I am not criticizing the whistleblower,” Maguire said. “... I am in no position to tell the committee to do an investigation or not to do an investigation.”

Schiff vowed at the end "we are gonna find out" the backstory, including whether U.S. aid was tied in any way to Trump's investigation request, which the president denies.

The combative conclusion of the session reflected the high tensions and enormous stakes surrounding Maguire's appearance. The rare open hearing of the intelligence committee commanded Washington's political attention given the implications for the newly launched impeachment inquiry against Trump, related to the whistleblower complaint.

Shortly after the hearing ended, Trump fired back on Twitter, saying: “Adam Schiff has zero credibility. Another fantasy to hurt the Republican Party!”

He told reporters that the process is a “disgrace,” defended his Ukraine call as “absolutely perfect” and accused Democrats of bringing any legislative progress to a standstill.

“They’re frozen, the Democrats,” he said.

Maguire's reputation was also on trial Thursday. He has come under fire from Democrats for initially withholding a copy of the complaint from lawmakers. That standoff was technically resolved when the complaint was declassified, and released to the public, in advance of the hearing. But Maguire was still pressed for answers from Democratic committee members on why his office did not hand over the complaint at first.

Maguire insisted he is "not partisan" and "not political," reminding the panel that he served under eight presidents and has taken the oath of office 11 times.

"I have served and led through turbulent times," he said. "... My integrity has never been questioned until now."

However, he maintained, "I believe that I handled this matter in full compliance with the law at all times.”

At one point in the hearing, during a tense exchange with Rep. Eric Swalwell, D-Calif., about allegations in the complaint of a potential “cover-up” regarding the Ukraine call, Maguire pushed back again by reminding the committee that the complaint was not based on direct knowledge.

“This is second-hand information from a whistleblower. I have no knowledge if that is a true and accurate statement,” Maguire said regarding the “cover-up” claim. The complaint said White House officials tried to “lock down” records of the call. In the same document, the whistleblower also acknowledged not being a “direct witness” to most of the events described, instead citing other U.S. officials who are not named.

Republican members also jumped to Maguire's defense at the hearing, while accusing Democrats of launching an "information warfare operation" against Trump. Rep. Chris Stewart, R-Utah, said of Maguire: "I think you're nuts if you're going to convince the American people that your cause is just by attacking this man, by impugning his character."

Schiff, meanwhile, questioned Maguire over the timeline of his cooperation with the committee. Maguire said he was not able to release the document at first because of executive privilege concerns and other issues. He also said he is not aware of any instance where such a complaint dealt with such "complicated and sensitive issues."

"I believe that this matter is unprecedented," he testified. He also said he does not know the identity of the whistleblower, but said he or she acted in "good faith."

Maguire maintained that he was "not stonewalling" despite the delay in the release of the complaint to the committee, but following "the law the way it was written, not the law as people might wish it to be." Maguire again explained that he was required to review the complaint for executive privilege.

"It took longer than I would have liked," he said. "But once the transcripts were released, no longer was there an issue of executive privilege ... so then I was able to release the information to you."

He added: "At no time did I ever try to withhold information."

The complaint cites “multiple White House officials with direct knowledge of the call" between Trump and Ukrainian President Volodymyr Zelensky, and accuses Trump of attempting “to pressure the Ukranian leader to take actions to help the President’s 2020 reelection bid.” It also presents allegations that the White House tried to "restrict access" to records of the call.

The complaint goes on to allege that White House officials said they were "directed" by White House attorneys to remove the transcript of the call from the computer system where they are normally kept. Instead, the transcript was allegedly kept on a different system normally used for classified information.

WHISTLEBLOWER COMPLAINT HAS BEEN DECLASSIFIED AND CONTAINS NO 'SURPRISES,' GOP LAWMAKER SAYS

Meanwhile, Republicans on the committee, including Ranking Member Devin Nunes, R-Calif., accused Democrats of orchestrating a public spectacle.

"I want to congratulate the Democrats on the rollout of their latest information warfare operation against the president, and their extraordinary ability once again to enlist the mainstream media in their campaign," Nunes said, adding that “we’ve been treated to an unending parade of press releases, press conferences, and fake news stories.”

"The released transcript of the call has already debunked that central assertion, but that didn't matter," he continued. "The Democrats simply moved the goalposts and began claiming that there doesn't need to be a quid pro quo for this conversation to serve as the basis for impeaching the president."

The White House, on Wednesday, released an unclassified version of the transcript of that controversial phone call. The memo, which does not reflect a “verbatim transcript” but is based on “notes and recollections” of those memorializing the call, shows Trump congratulating Zelensky on his election victory before Trump eventually gets to the subject of the Bidens, seeking an investigation of some kind.

Maguire says that the whistleblower complaint released Thursday "is in alignment with what was released yesterday by the president."

Trump first asked Zelensky to do a "favor" and “find out what happened” with the hacking of the DNC server in 2016. He referenced CrowdStrike, a cyber firm used by the DNC to investigate the attacks.

Then, Trump went on to seek assistance in investigating the Bidens.

“The other thing, there’s a lot of talk about Biden’s son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney General would be great,” Trump said. “Biden went around bragging that he stopped the prosecution so if you can look into it…It sounds horrible to me.”

This refers to Joe Biden, while vice president, urging Ukraine to fire its top prosecutor, Viktor Shokin, who was investigating the natural gas firm Burisma Holdings — where Hunter Biden was on the board. Biden has maintained that corruption concerns prompted his intervention.

DEMS DOUBLE DOWN ON IMPEACHMENT AFTER UKRAINE TRANSCRIPT RELEASE

While the transcript shows Trump pressing Ukraine to “look into” the Bidens, it does not show the U.S. president explicitly linking that request to U.S. aid — which had been frozen days earlier, a detail that fueled impeachment calls earlier this week and helped prompt House Speaker Nancy Pelosi to announce a formal impeachment inquiry Tuesday.

Hours after the transcript was released, Trump met with Zelensky on the sidelines of the United Nations General Assembly in New York City. Reporters asked Zelensky about the call with Trump, and whether he felt pressured by the U.S. president, as alleged by the whistleblower—an accusation he denied.

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“I think you read everything. I’m sorry but I don’t want to be involved in democratic elections of the USA,” he said. “We had a good phone call. It was normal, we spoke about many things.”

He added: “I think you read it that nobody pushed me.”

Republicans have also raised concerns about the whistleblower's possible "political bias," as the intelligence community inspector general found indications the whistleblower was "in favor of a rival political candidate" of Trump.

Fox News' Chad Pergram, Gregg Re, Ed Henry, and Jake Gibson contributed to this report.