Congressional Democrats on Monday subpoenaed four aides to Secretary of State Mike Pompeo as part of their bicameral investigation into the State Department's firing of its inspector general earlier this year.

House Foreign Affairs Committee Chairman Eliot Engel of New York, House Oversight Committee Chair Carolyn Maloney, also of New York, and Senate Foreign Relations Ranking Member Bob Menendez, of New Jersey, announced the subpoenas compelling four Pompeo aides to testify as part of their joint investigation into the firing of former State Department Inspector General Steve Linick.

GRASSLEY ASKS TRUMP FOR 'EXPLANATION' ON FIRING OF TOP STATE DEPT. WATCHDOG TO ENSURE IT'S NOT FOR 'POLITICAL REASONS'

"The Administration continues to cover up the real reasons for Mr. Linick's firing by stonewalling the Committees' investigation and refusing to engage in good faith," they said in a joint statement. "That stonewalling has made today's subpoenas necessary, and the Committees will continue to pursue this investigation to uncover the truth that the American people deserve."

The subpoenas were directed to Under Secretary of State for Management Brian Bulatao, acting State Department Legal Adviser Mark String, Deputy Assistant Secretary of State for Political-Military Affairs Michael Miller, and Pompeo's senior adviser, Toni Porter.

The subpoenas come as the committees investigate why President Trump fired Linick from his post.

Linick, at the time of his firing, had been investigating the State Department's $7 billion arms sale to Saudi Arabia, despite congressional objections. Linick also was probing whether Pompeo misused department funds for personal errands such as walking his dog, making dinner reservations and picking up his dry cleaning.

Linick defended his work as "impartial" and "without regard to politics," during his closed-door testimony on Capitol Hill in June.

FIRED STATE DEPARTMENT IG DEFENDS ROLE

Linick's removal continued a series of changes among the government's inspectors general. The most notable was Trump's April firing of Inspector General for the Intelligence Community Michael Atkinson, who played a role in handling the whistleblower complaint that led to the Ukraine probe -- and Trump's subsequent impeachment.

Meanwhile, the Democrats also announced Monday that State Department Executive Secretary Lisa Kenney, during her Senate confirmation hearing to be U.S. Ambassador to Peru, committed to appear for a staff-led, transcribed interview conducted by the three committees on Aug. 7.

They also released highlights of an interview they conducted with former State Department official Charles Faulkner.

"We thank Mr. Faulkner for appearing voluntarily last week. His testimony raises additional questions about the Administration's decision to sell arms to Saudi Arabia and the UAE under a spurious 'emergency declaration' in 2019 – questions that we understand the State Department Inspector General's office sought answers to prior to Mr. Linick's firing," they wrote, adding that his testimony "depicts a small group of senior State Department officials determined to ignore legitimate humanitarian concerns among their ranks and on Capitol Hill in order to ram through more than $8 billion in arms sales to Gulf countries."

The committee added that Faulkner "presented a timeline for these events that refutes a senior official's testimony to Congress about when these decisions were made."

The Democrats committed to releasing the full transcript of Faulkner's interview "as soon as possible."

Fox News' Jason Donner contributed to this report.