Michael Atkinson, the intelligence community watchdog who handled the whistleblower complaint that led to President Trump's impeachment, issued a two-page statement on his removal from the role Sunday night. In the statement, Atkinson said he had "faithfully discharged" his duties as inspector general and spent his nearly two-decade career serving "without regard to partisan favor or political fear."

Mr. Trump announced less than 48 hours earlier that he had relieved Atkinson, whom the president had appointed to serve as Intelligence Community Inspector General in 2018, of his duties because Mr. Trump "no longer" had confidence in him.

"It is hard to not to think the president's loss of confidence in me derives from my having faithfully discharged my legal obligations as an independent and impartial Inspector General, and from my commitment to do so," Atkinson wrote in the statement.

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Atkinson wrote that he was "legally obligated" to make sure whistleblowers had confidentiality and were protected from reprisal. He also issued an appeal to other would-be whistleblowers working for the federal government.

Then-Intelligence Community Inspector General Michael Atkinson is seen arriving to testify at an October 4, 2019 House Intelligence Committee closed-door hearing on the whistleblower complaint about President Trump's dealings with Ukraine. Jonathan Ernst / REUTERS

"The American people deserve an honest and effective government. They are counting on you to use authorized channels to bravely speak up – there is no disgrace in doing so," Atkinson wrote. "Please do not allow recent events to silence your voices."

During a coronavirus task force briefing Saturday, Mr. Trump said that he knew the identity of the whistleblower and implied it was not a secret. He also insisted that, as president, he had the "absolute right" to fire Atkinson, saying "I thought he did a terrible job" and calling him "a disgrace."

"He took a whistleblower report that turned out to be fake, about conversation with Ukraine, took a report and brought it to Congress with an emergency," Mr. Trump said.

Mr. Trump also accused Atkinson, without evidence, of being partisan. Atkinson, who worked at the U.S. Department of Justice for over a decade, said Sunday that he has "spent my entire 17-year career as a public servant acting without regard to partisan favor or political fear."

Although by statute, Mr. Atkinson was to have served an additional 30 days after Congress was notified of his removal, he was instead placed on immediate administrative leave, according to a source familiar with the matter.

His removal sparked bipartisan concern from lawmakers who raised questions about the reason for his ouster.

On Saturday, Republican Senator Chuck Grassley said in a statement that Congress had been "crystal clear" that written reasons must be given when IGs were removed for a lack of confidence. "More details are needed from the administration," Mr. Grassley said.

Susan Collins, a Republican Senator from Maine, said Atkinson's removal "was not warranted."

Atkinson was involved in the early stages of what eventually became the impeachment inquiry against Mr. Trump. On August 12, 2019, a whistleblower filed a complaint about an interaction between Mr. Trump and Ukrainian President Volodymyr Zelensky.

According to the transcript of the call that was later released, the whistleblower said, "I have received information from multiple U.S. Government officials that the President of the United States is using the power of his office to solicit interference from a foreign country in the 2020 U.S. election. This interference includes, among other things, pressuring a foreign country to investigate one of the President's main domestic political rivals."

Federal law stipulates that the inspector general must investigate any report of an "urgent concern" by an employee of the intelligence community and determine whether it "appears credible" within two weeks. If it is, the inspector general must then report it to the director of national intelligence, whose office is responsible for overseeing the nation's 17 intelligence agencies.

Atkinson concluded that the complaint was credible enough to be considered an "urgent concern," and on August 26, he sent a letter to then-acting Director of National Intelligence Joseph Maguire, who left the position in February. In September, Atkinson wrote a letter to the House Intelligence Committee that revealed Maguire decided not to forward the complaint to Congress.

Atkinson's removal is the latest in a series of ousters at the top of the intelligence community ranks, most of which have been concentrated within the Office of the Director of National Intelligence. In February, Maguire was replaced by the current acting director, Richard Grenell, who serves concurrently as Ambassador to Germany. Maguire's then-second in command, career CIA official Andrew Hallman, was also removed.

Mr. Trump then re-nominated Republican Congressman John Ratcliffe of Texas, who previously withdrew his candidacy amid scrutiny of his national security credentials, to the permanent DNI position — though it is unclear when a confirmation process may begin.

Earlier this month, Grenell also announced the removal of the acting director of the National Counterterrorism Center (NCTC), career intelligence official Russ Travers, as well as his deputy. Mr. Trump nominated Christopher Miller, a former Army Special Forces officer, to replace Travers. The NCTC's top two positions have since been filled on an acting basis by two other career officials, pending Miller's confirmation.

ODNI said over the weekend that the new acting intelligence community inspector general will be Thomas Monheim, a career intelligence official who was most recently General Counsel of the National Geospatial-Intelligence Agency.

Kathryn Watson and Grace Segers contributed to this report.