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Sen. Chuck Grassley, R-Iowa, is demanding President Trump explain to Congress why he fired the State Department Inspector General Steve Linick.

Linick was removed from his post on Friday in the midst of investigating the State Department's $7 billion arms sale to Saudi Arabia despite congressional objections.

The former IG was also probing whether Secretary of State Mike Pompeo misused department funds for personal errands such as walking his dog, making dinner reservations and picking up his dry cleaning.

Trump told Congress he made the decision to fire Linick because he “no longer” had the “fullest confidence” in him, but Grassley said in a letter on Monday that reasoning was not sufficient and pushed him to give a fuller explanation by June 1.

"Congress’s intent is clear that an expression of lost confidence, without further explanation, is not sufficient to fulfill the requirements of the IG Reform Act," Grassley wrote. "This is in large part because Congress intended that inspectors general only be removed when there is clear evidence of unfitness, wrongdoing, or failure to perform the duties of the office."

Grassley also expressed concern at another unanswered letter about the reason for removing the previous inspector general Michael Atkinson, which Congress requested more information on since April 8. Atkinson was fired for his handling of a whistleblower complaint that was the basis for impeachment proceedings against Trump.

Linick is the fourth government watchdog to be removed in the course of six weeks.

He was replaced by Ambassador Stephen Akard, who is the director of the Office of Foreign Missions and has close ties to Vice President Pence.

Grassley pointed out that this new appointment could create an "obvious conflicts of interest" that "unduly threaten the statutorily required independence of inspectors general."

"I want to work with you to ensure that the enemy here is wasteful government spending, not the government watchdogs charged with protecting the taxpayer," Grassley wrote.

Trump defended Pompeo on Monday, dismissing allegations of wrongdoing and saying, "They're bothered he was having somebody walk his dog? This is what you get with the Democrats."

Democrats have argued that Linick's removal was retaliatory because of his investigative role, and Grassley warned IGs should not be removed for "political reasons."

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"IGs are intended to be equal opportunity investigators and are designed to combat waste, fraud, abuse, and misconduct without regard to political affiliation. They are the ultimate swamp drainers," he wrote. "Removal of IGs without explanation could create a chilling effect in the oversight community, and risks decreasing the quantity, quality, fidelity, and veracity of their reports."