Trump said recently that he fired Atkinson for doing a "terrible job" with a whistleblower report that kicked off the president's impeachment. Atkinson said he was likely fired for "having faithfully discharged my legal obligations as an independent and impartial inspector general."

The letter was first reported by Bloomberg.

Grassley's office did not comment other than to point to a Tuesday statement reiterating his support for inspectors general and urging the president to use them. Grassley also said he would seek more explanation from Trump in the immediate aftermath of Atkinson's firing. He's also been a longtime advocate for inspectors general and whistleblowers.

“Inspectors general provide a critical check on an otherwise unaccountable bureaucracy. In other words, they help drain the swamp. Their duty is to provide nonpartisan recommendations and remove politics from the inner workings of our federal government. The White House should empower inspectors general so they’re able to do their job," Grassley said, citing recent IG work on surveillance of the Trump campaign in the 2016 election.

He also hinted that Trump owes the public more information.

"President Trump must prioritize filling inspector general vacancies with permanent nominees so this important congressional oversight function can occur," Grassley said.

Trump fired Glenn Fine, a Defense Department watchdog, on Monday. Grassley thanked "Fine for his service to his country" in his statements.