The top Democrats on the House and Senate foreign affairs committees launched an investigation Saturday into President Trump's Friday night firing of State Department Inspector General (IG) Steve Linick.

Rep. Eliot Engel, the chair of the House Committee on Foreign Affairs, and Sen. Bob Menendez, the lead Democrat on the Senate Committee on Foreign Relations, wrote letters to the White House, State Department and the Office of the Inspector General (OIG) to preserve all records related Linick's dismissal and to turn the information over by Friday, May 22.

“President Trump’s unprecedented removal of Inspector General Linick is only his latest sacking of an inspector general, our government’s key independent watchdogs, from a federal agency. We unalterably oppose the politically-motivated firing of inspectors general and the president’s gutting of these critical positions,” Engel and Menendez wrote in the letter to the White House.

Sen. Bob Menendez, D-N.J., and Rep. Eliot Engel, D-N.Y., are investigating Trump's firing of the State Department Inspector General Steve Linick on Friday, May 15, 2020.

The lawmakers suggest that the firing "may be an illegal act of retaliation" because the IG was investigating Secretary of State Mike Pompeo.

TRUMP FIRES STEVE LINICK, OBAMA APPOINTEE WHO BRIEFED CONGRESS ON BIDEN-UKRAINE TIES

“Reports indicate that Secretary Pompeo personally made the recommendation to fire Mr. Linick, and it is our understanding that he did so because the Inspector General had opened an investigation into wrongdoing by Secretary Pompeo himself," the lawmakers write. "Such an action, transparently designed to protect Secretary Pompeo from personal accountability, would undermine the foundation of our democratic institutions and may be an illegal act of retaliation."

It was Pompeo who suggested to Trump that he fire Linick, two Trump Administration officials told Fox News on Saturday.

“Secretary Pompeo recommended the move, and President Trump agreed," a White House official told Fox News.

FILE - In this Dec. 10, 2014, file photo Steve Linick, State Department Inspector General, testifies on Capitol Hill in Washington. A senior department official said President Donald Trump removed Linick from his job as State Department’s inspector general on Friday, May 15, 2020, but gave no reason for his ouster. (AP Photo/Evan Vucci, File)

A Democratic congressional aide told Fox News that Linick was investigating possible misuse of a political appointee at the State Department to perform personal tasks for Pompeo and his wife.

Trump fired Linick on Friday night, saying in a letter to Congress that he no longer had confidence in the State Department IG -- who was appointed during the Obama administration and had overseen reports critical of the department’s policies since Trump took office.

Fox News learned in October 2019 that Linick had hosted a closed-door briefing on the Ukraine investigation for congressional committee aides that examined communications between Trump lawyer Rudy Giuliani and fired Ukrainian prosecutor Viktor Shokin and current Prosecutor-General Yuriy Lutsenko.

Before that, his office had raised concerns about then-Secretary of State Hillary Clinton’s use of a private email server.

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Linick’s removal continues a series of changes among the government’s inspectors general. The most notable of which was Trump’s April firing of then-Inspector General for the Intelligence Community Michael Atkinson for his role in the whistleblower complaint that led the Ukraine probe -- and Trump’s subsequent impeachment.

Sen. Chuck Grassley is the chairman of the Senate Judiciary Committee. (AP Photo/Andrew Harnik)

Sen. Chuck Grassley, a longtime advocate for inspectors general who denounced Atkinson's ouster, joined Democrats Saturday in criticizing Trump's latest IG firing.

Grassley, R-Iowa, said the 2008 Inspector General Reform Act requires the president to provide Congress with a written explanation at least 30 days prior to removing an inspector general -- and Trump didn't meet this responsibility.

"As I’ve said before, Congress requires written reasons justifying an IG’s removal," Grassley said in a statement. "A general lack of confidence simply is not sufficient detail to satisfy Congress."

Linick will be replaced by Stephen Akard, a former career foreign service officer with close ties to Vice President Pence, a Trump administration official told The Associated Press.

The Democrats are not only seeking documents and communications related to Linick's ousting but also to the qualifications of Akard and information on all the pending IG investigations Linick's office was working on before his untimely removal.

Democrats have panned Trump's purge of inspectors general as a way for his Administration to avoid accountability, and they've sought to strengthen the role of the independent watchdogs in newly passed legislation.

“The President’s late-night, weekend firing of the State Department Inspector General has accelerated his dangerous pattern of retaliation against the patriotic public servants charged with conducting oversight on behalf of the American people," House Speaker Nancy Pelosi said Friday night. "Inspector General Linick was punished for honorably performing his duty to protect the Constitution and our national security, as required by the law and by his oath."

Fox News' Rich Edson, David Spunt, Adam Shaw and Dom Calicchio contributed to this report.