U.S. Democrats have launched an investigation into President Donald Trump's dismissal of state department internal surveillance.
Inspector General Steve Linick was investigating Secretary of State Mike Pompeo for suspected abuse of power, reports say.
But he was fired on Friday night after Trump said he no longer commanded his full confidence.
The move provoked angry criticism from senior Democrats in Congress.
They accused Trump of retaliating for public officials who want to hold his government accountable. Linick was the third official responsible for monitoring the government's misconduct to be fired in recent weeks.
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The former prosecutor was appointed by Trump's predecessor Barack Obama to oversee spending and detect maladministration in the state department.
On Saturday, top Democrats on the House and Senate foreign affairs committees questioned the timing of Linick's removal and announced an immediate investigation.
"We are unalterably opposed to the dismissal of general inspectors for political reasons and to the eviction of the president from these critical positions," Congressman Eliot Engel and Senator Bob Menendez said in a statement.
They said Linick "opened an investigation into irregularities by Secretary Pompeo himself", adding that his resignation was "designed in a transparent manner to protect Secretary Pompeo from personal liability".
Linick began investigating allegations that Pompeo had misused employees to perform personal tasks, the American media reported.
Engel and Menendez asked the White House and the State Department to hand over all records related to his resignation next Friday.
Meanwhile, on Saturday, the White House said the decision to fire Linick was prompted by Pompeo himself. "Secretary Pompeo recommended the change, and President Trump agreed," said an official.
What happened on Friday?
Trump sent a letter to House Speaker Nancy Pelosi in which she declared her intention to fire Linick.
Under federal law, the Trump administration must notify Congress 30 days in advance of its plans to fire an inspector general. Linick is expected to step down after that period, with some reports suggesting that a Trump political ally is being lined up to replace him.
"It is vital that I have complete confidence in the nominees who serve as inspectors general. This is no longer the case with this inspector general," Trump said in the letter.
Shortly after the announcement of Linick's resignation, Engel, chairman of the Chamber's Foreign Affairs Committee, said that Linick had opened an investigation into Pompeo.
"Linick's dismissal amid such an investigation strongly suggests that this is an illegal act of retaliation," he said in a statement.
House Speaker Nancy Pelosi said that Linick was "punished for honoring his duty to protect the constitution and our national security".
It was the latest in a series of layoffs by independent government watchmen.
Last month, Trump fired Michael Atkinson, the inspector general of the intelligence community.
Atkinson warned Congress for the first time about a complaint that led to Trump's impeachment trial.