Washington — A bipartisan group of senators are demanding President Trump provide a more thorough explanation for ousting Michael Atkinson, the intelligence community's inspector general who handled the whistleblower complaint that sparked the House's impeachment inquiry into the president and his dealings last year with Ukraine.

Led by Senator Chuck Grassley, a Republican from Iowa, the eight GOP and Democratic senators sent a letter to Mr. Trump on Wednesday reiterating the statutory requirements for removing the inspector general and raising concerns about his decision to place Atkinson on administrative leave immediately.

"As supporters of the inspector general community, and as advocates for government transparency and accountability, it is our responsibility to confirm that there are clear, substantial reasons for removal," the senators wrote in their letter.

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Mr. Trump fired Atkinson last week and informed the leaders of the Senate Intelligence Committee that the inspector general "no longer" had his "fullest confidence." Atkinson was placed on immediate administrative leave, and Thomas Monheim, general counsel of the National Geospatial-Intelligence Agency, was selected as the acting inspector general of the intelligence community.

Federal statute requires the president to inform Congress of the reasons for removing the intelligence community's internal watchdog at least 30 days before doing so, but the senators said Mr. Trump's reasoning — a loss of confidence — was not adequate.

"Congressional intent is clear that an expression of lost confidence, without further explanation, is not sufficient to fulfill the requirements of the statute," the lawmakers said. "This is in large part because Congress intended that inspectors general only be removed when there is clear evidence of wrongdoing or failure to perform the duties of the office, and not for reasons unrelated to their performance, to help preserve IG independence."

The senators also said the president had effectively bypassed Congress by removing Atkinson immediately and tapping Monheim as the acting inspector general.

"By placing the IG on 30 days of administrative leave and naming an acting replacement, the administration has already effectively removed that IG and appears to have circumvented Congress's role in this process," the group of senators said.

Mr. Trump has until April 13 to provide more detailed reasoning for Atkinson's removal, as well as an explanation for how the appointment of an active watchdog fulfills statutory requirements.

In addition to Grassley, the signatories are: Republican Senators Susan Collins of Maine and Mitt Romney of Utah, and Democratic Senators Gary Peters of Michigan, Mark Warner of Virginia, the top Democrat on the Intelligence Committee, Ron Wyden of Oregon, Dianne Feinstein of California and Jon Tester of Montana.

Congressional Democrats lambasted the president's decision to fire Atkinson, especially as the move came in the midst of the coronavirus pandemic. Republicans, meanwhile, praised Atkinson for his work.

While it was Mr. Trump who nominated Atkinson to serve as the intelligence community inspector general, his removal was seen as part of an effort by the White House to target those involved in the impeachment process.

The president first discussed removing Atkinson last fall after it was revealed that a whistleblower had filed a complaint with his office. Atkinson concluded the complaint, which centered around a July phone call between Mr. Trump and Ukrainian President Volodymyr Zelensky, was credible and reported it to the director of national intelligence in late August as required under federal law. Atkinson then notified the House Intelligence Committee in September that then-acting Director of National Intelligence Joseph Maguire, who is no longer in the role, decided not to forward the whistleblower's complaint to Congress.

The House impeached Mr. Trump on charges of abuse of power and obstruction of Congress related to his dealings with Ukraine in December, and the Senate acquitted him on both charges in February.

Atkinson addressed his firing in a two-page statement Sunday, saying it was "hard 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."

He also urged government whistleblowers not to be cowed by recent events, saying they should not allow them "to silence your voices."

Mr. Trump and his allies in Congress have repeatedly tried to reveal the name of the alleged whistleblower who filed the complaint with Atkinson's office, though the person's identity remains unknown.