The White House and the State Department did not provide an "adequate response" to lawmakers' requests for documents related to the firing of the department's inspector general, a congressional aide said on Friday.
"But pressure is building and the truth will come out," a House of Representatives Foreign Relations Committee aide said.
The White House declined to comment. The State Department did not immediately respond to a request for comment.
President Donald Trump announced earlier this month that he planned to remove department watchdog Steve Linick, saying he had lost confidence in him.
Congressional Democrats immediately launched a probe into the firing, saying it had been their understanding that Linick was investigating wrongdoing by U.S. Secretary of State Mike Pompeo himself.
Eliot Engel, chairman of the House Foreign Affairs Committee, and Senator Bob Menendez, the top Democrat on the Senate's Foreign Relations panel, said Linick had been probing Trump's declaration of a national emergency last year to clear the way for $8 billion in military sales, mostly to Saudi Arabia.
Congressional aides have also said Linick was looking into whether Pompeo misused a taxpayer-funded political appointee to perform personal tasks for himself and his wife, such as walking their dog.
Pompeo on Wednesday told reporters he wished he had recommended to Trump that Linick be fired "some time ago" but denied that he urged the department's internal watchdog be fired in retaliation for internal investigations he was conducting.
Trump has repeatedly balked at Congress' power to oversee his administration.
He has fired or removed three other inspectors general in recent months: Christi Grimm, the watchdog for Health and Human Services, after a report on American hospitals suffering shortages amid the coronavirus outbreak; Michael Atkinson, who was involved in triggering the impeachment investigation into Trump; and Glenn Fine, who was to oversee the government's COVID-19 response.
U.S. law allows a president to remove inspector generals, who serve as watchdogs to uncover improper activities within government agencies, but Democratic lawmakers have called it part of a pattern of retaliation.
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