Senator Bob Menendez of New Jersey and representative Eliot Engel have launched an investigation into president Donald Trump’s firing of inspector general Steve Linick.
The announcement of the investigation was tweeted by Mr Menendez on Saturday afternoon with an open letter from the two congressmen.
The lawmakers request that the White House and State Department preserve and produce all records relating to Friday night’s late night firing.
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A senior department official said Mr Trump removed Mr Linick from his job on Friday but gave no specific reason.
In a letter to Congress, the president said Mr Linick no longer had his full confidence and that his removal would take effect in 30 days.
He did not mention Mr Linick by name in the letter.
Senator Menendez is the ranking member of the Senate Committee on Foreign Relations, whereas Mr Engel is the chairman of the House Committe on Foreign Affairs.
They have asked for all records to be turned over to both committees by Friday 22 May.
Describing the firing as “unprecedented”, the two lawmakers say that they “unalterably oppose the politically-motivated firing of inspectors general and the president’s gutting of these critical positions”.
“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 has opened an investigation into wrongdoing by secretary Pompeo himself,” they continue.
They add that such an action to protect Mr Pompeo from personal accountability, not only undermines the foundation of our democratic institutions, but may also be an illegal act of retaliation.
Their concern is amplified by the fact that the firing came just hours after the House passed the Heroes Act, which contains additional legal protections for inspectors general.
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Separately, the house speaker, Nancy Pelosi, warned of an acceleration in the “dangerous pattern of retaliation” against government watchdogs or oversight officials.
Mr Linick, who was appointed by then president Barack Obama in 2013, will be replaced by Stephen Akard, a State Department spokesperson said on Friday.