Sen. Chuck Grassley (R-IA) and seven other senators on both sides of the aisle sent a letter to President Donald Trump on Wednesday asking him to fully explain to Congress why he fired Michael Atkinson, the inspector general for the intelligence community, last week.

The other signatories were Sens. Gary Peters (D-MI), Susan Collins (R-ME), Dianne Feinstein (D-CA), Jon Tester (D-MT), Mitt Romney (R-UT), Mark Warner (D-VA), and Ron Wyden (D-OR).

The senators pointed out that Trump was legally required to give the Senate and House Intelligence Committees his reasons in detail for firing an inspector general, and that the law mandates he do so 30 days in advance.

However, Trump only told the Senate Intelligence Committee that he no longer had “the fullest confidence” in Atkinson, and the inspector general had already been fired by then.

“Congressional intent is clear that an expression of lost confidence, without further explanation, is not sufficient to fulfill the requirements of the statute,” the senators wrote. “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 lawmakers requested that Trump respond to their request “no later than” Monday on April 13.

“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,” they wrote.

Though Grassley often falls in line with Trump and his agenda, the Iowa Republican (who has long worked to protect whistleblowers and those overseeing government accountability) notably pushed back against him several times when the President persistently attempted to unmask the whistleblower who complained about his call with the Ukrainian president.

Trump fired Atkinson on Friday. The next day, he called the inspector general a “disgrace to IGs” and griped about Atkinson passing on the aforementioned whistleblower complaint to his higher-ups.

A few days after his ouster, Atkinson accused Trump of targeting him for handling the complaint impartially.