The US state department's inspector general, Steve Linick, has become the latest senior official to be fired by US President Donald Trump.
Mr Trump said Mr Linick no longer had his full confidence and that he would be removed in 30 days.
Mr Linick had begun investigating Secretary of State Mike Pompeo for suspected abuse of office, reports say.
Democrats say Mr Trump is retaliating against public servants who want to hold his administration to account.
"It is vital that I have the fullest confidence in the appointees serving as inspectors general. That is no longer the case with regard to this inspector general," Mr Trump is quoted as saying in a letter sent late on Friday to House Speaker Nancy Pelosi, US media report.
Not long after Mr Linick's dismissal was announced, the chairman of the House Foreign Affairs Committee said Mr Linick had opened an investigation into Secretary of State Mike Pompeo.
"This firing is the outrageous act of a president trying to protect one of his most loyal supporters, the secretary of state, from accountability," Eliot Engel, a Democrat, said in a statement.
"I have learned that the Office of the Inspector General had opened an investigation into Secretary Pompeo. Mr Linick's firing amid such a probe strongly suggests that this is an unlawful act of retaliation."
Mr Engel did not provide any further details about the content of this investigation into Mr Pompeo.
Mr Linick was examining complaints that Mr Pompeo had improperly used staff for personal tasks, such as picking up dry cleaning and walking his dog, according to US media.
Mr Linick, a former prosecutor, was appointed by Mr Trump's predecessor, Barack Obama, to oversee spending and detect mismanagement at the state department.
Democrats have been reacting to the move. House Speaker Nancy Pelosi said Mr Linick was "punished for honourably performing his duty to protect the constitution and our national security".
The late-night, weekend firing of State Department IG Steve Linick is an acceleration of the President’s dangerous pattern of retaliation against the patriotic public servants charged with conducting oversight on behalf of the American people. https://t.co/VavmuJpX25— Nancy Pelosi (@SpeakerPelosi) May 16, 2020
"The president must cease his pattern of reprisal and retaliation against the public servants who are working to keep Americans safe, particularly during this time of global emergency," she added in a statement.
Senator Chris Murphy, a Democrat from Connecticut, said the Senate Foreign Relations Committee needed to learn more about the dismissal.
If Inspector General Linick was fired because he was conducting an investigation of conduct by Secretary Pompeo, the Senate cannot let this stand.— Chris Murphy (@ChrisMurphyCT) May 16, 2020
The Senate Foreign Relations Committee must get to bottom of what happened here.
This is the latest in a series of dismissals of independent government watchdogs.
Mr Atkinson first alerted Congress to a whistleblower complaint that led to Mr Trump's impeachment trial.