President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on Thursday lashed out at House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) and a whistleblower who raised concerns about his interactions with the president of Ukraine following testimony about the complaint from the acting director of national intelligence.

In a series of tweets, Trump questioned the whistleblower's credibility and tore into Schiff, who oversaw a hearing Thursday morning with acting Director of National Intelligence Joseph Maguire that focused on the whistleblower's complaint related to Trump's July 25 call with the Ukrainian president.

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"Liddle’ Adam Schiff, who has worked unsuccessfully for 3 years to hurt the Republican Party and President, has just said that the Whistleblower, even though he or she only had second hand information, 'is credible,'" Trump tweeted. "How can that be with zero info and a known bias. Democrat Scam!"

Liddle’ Adam Schiff, who has worked unsuccessfully for 3 years to hurt the Republican Party and President, has just said that the Whistleblower, even though he or she only had second hand information, “is credible.” How can that be with zero info and a known bias. Democrat Scam! — Donald J. Trump (@realDonaldTrump) September 26, 2019 The president blasted the whistleblower for filing the complaint based on "second hand information," calling the document released earlier in the day "Another Fake News Story!"

A whistleblower with second hand information? Another Fake News Story! See what was said on the very nice, no pressure, call. Another Witch Hunt! — Donald J. Trump (@realDonaldTrump) September 26, 2019

But Schiff was the president's main target. The president blasted the Democratic lawmaker as having "zero credibility" in a tweet, then angrily went after the congressman upon landing at Joint Base Andrews following three days at the United Nations.

"Adam Schiff has zero credibility. Another fantasy to hurt the Republican Party!" Trump tweeted as he arrived back in Washington following three days at the United Nations.

Adam Schiff has zero credibility. Another fantasy to hurt the Republican Party! — Donald J. Trump (@realDonaldTrump) September 26, 2019

"We’ve done so many things that are so incredible ... and I have to put up with Adam Schiff on an absolutely perfect phone call to the new president of Ukraine," Trump said.

He went on to bemoan that Schiff wouldn't look into Democratic presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE over his son's dealings in Ukraine.

Schiff, asked about the president's tweet, quipped to reporters, "I'm always flattered when I'm attacked by someone [of] the president's character."

The House this week launched a formal impeachment inquiry into Trump amid scrutiny of the president's conduct during a July 25 call with the leader of Ukraine.

The White House on Wednesday released a rough transcript that showed Trump urged the Ukrainian president to look into Biden for his role in the ouster of a Ukrainian prosecutor in 2016 who was accused of overlooking corruption.

The whistleblower complaint was made public Thursday morning and alleged that the White House sought to keep the contents of the call secret using abnormal protocols.

Maguire testified before the House Intelligence Committee about the complaint and about allegations that the White House had withheld it. He further testified that he found the whistleblower to be "credible," and that Trump had not asked about the identity of the individual.

Trump has consistently attacked the whistleblower and questioned their motives while acknowledging he does not know their identity. In a tweet earlier this week, he questioned whether the individual was "on our country's side" and has decried the person as a "partisan hack."

The New York Times reported Thursday that Trump told U.S. diplomatic staff that he thought whoever provided information to the whistleblower was "close to a spy."

— Cristina Marcos contributed