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Republican Sen. Chuck Grassley of Iowa said Wednesday he's assembled a bipartisan group of senators calling on President Donald Trump to provide a "detailed written explanation" for the removal of the inspector general who flagged the Ukraine whistleblower complaint.

Citing a loss of confidence, Trump told Congress on Friday that he was removing Michael Atkinson, the inspector general of the intelligence community, who deemed the complaint an "urgent concern" and was then required by law to provide it to the congressional intelligence committees.

The complaint and a House inquiry ended in Trump's impeachment. He was acquitted by the Senate in February.

In a letter sent to Trump on Wednesday, Grassley says current law requires the president to notify Congress in writing of the reasons for removal 30 days in advance, and it also notes reports that Atkinson had already been placed on administrative leave, which effectively sidestepped the requirement.

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"Congressional intent is clear that an expression of lost confidence, without further explanation, is not sufficient to fulfill the requirements of the statute," the letter reads.

"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 letter continues.

Grassley, a co-founder and leader of the Whistleblower Protection Caucus, said senators from both parties raised concerns with the administration of President Barack Obama in 2009 after an inspector general was placed on leave.

Also signing the letter were Republican Sens. Mitt Romney of Utah and Susan Collins of Maine and Democratic Sens. Gary Peters of Michigan, Mark Warner of Virginia, Ron Wyden of Oregon, Dianne Feinstein of California and Jon Tester of Montana.

The letter asks Trump to provide a "more detailed reasoning" for Atkinson's removal by Monday. "Please also provide your views on how the appointment of an acting official prior to the end of the 30 day notice period comports with statutory requirements," the letter reads.

Atkinson is a Trump appointee who was confirmed by the Senate in 2018. He is a career Justice Department prosecutor.

Trump said of Atkinson on Saturday that "I thought he did a terrible job, absolutely terrible," and that "he took a fake report and brought it to Congress. ... Not a big Trump fan, that I can tell you."

Atkinson defended his handling of the complaint Sunday. He said in a statement that it was hard not to think Trump fired him because he "faithfully discharged my legal obligations as an independent and impartial" inspector general.

The senators wrote: "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."