Three top House Democrats on Thursday launched an investigation into what they described as “unprecedented firings” at the Department of Homeland Security.

House committee chairmen Elijah Cummings Elijah Eugene CummingsBlack GOP candidate accuses Behar of wearing black face in heated interview Overnight Health Care: US won't join global coronavirus vaccine initiative | Federal panel lays out initial priorities for COVID-19 vaccine distribution | NIH panel: 'Insufficient data' to show treatment touted by Trump works House Oversight Democrats to subpoena AbbVie in drug pricing probe MORE (D-Md.), Jerrold Nadler Jerrold (Jerry) Lewis NadlerDemocrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (D-N.Y.) and Bennie Thompson Bennie Gordon ThompsonHouse panel pans ICE detention medical care, oversight Senate to hold nomination hearing for Wolf next week Hillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers MORE (D-Miss.) wrote a letter to acting Homeland Security Secretary Kevin McAleenan demanding communications related to the departure or potential departure of former Homeland Security Secretary Kirstjen Nielsen Kirstjen Michele NielsenMore than million in DHS contracts awarded to firm of acting secretary's wife: report DHS IG won't investigate after watchdog said Wolf, Cuccinelli appointments violated law Appeals court sides with Trump over drawdown of immigrant protections MORE, former Secret Service Director Randolph Alles and other officials.

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“We are deeply concerned that the firing and forced resignation of these officials puts the security of the American people at risk,” the lawmakers wrote. “We are also concerned that the President may have removed DHS officials because they refused his demands to violate federal immigration law and judicial orders.”

“Moreover, we are concerned by reports that, even as he has removed the Department’s leadership, the President has sought to empower a White House aide, Stephen Miller Stephen MillerSpecial counsel investigating DeVos for potential Hatch Act violation: report Trump confirms another White House staffer tested positive for COVID-19 Biden pick creates furor, underscoring bitterness over Obama immigration policy MORE, to ‘be in charge of handling all immigration and border affairs,'” the Democrats wrote, quoting from a recent report in The Washington Post.

The announcement of the probe comes one day after the White House declined to make Miller available to testify before Congress.

The letter cites reports that Miller and other senior officials advocated for Nielsen’s departure after she identified legal hurdles to the president’s effort to restart family separations at the U.S. southern border. Trump abruptly announced Nielsen’s resignation on Twitter on April 7, after months of rumors the president would force her out.

The Democrats pointed to other recent departures at the Department of Homeland Security, including the removal of Alles and resignations of Undersecretary for Management Claire Grady and acting Director of Immigrations and Customs Enforcement (ICE) Ronald Vitiello.

Trump withdrew Vitiello’s nomination to be permanent ICE Director days before he resigned, telling reporters he wanted to go in a “tougher direction.” Miller, who is known for his hardline immigration views, is said to have advocated for withdrawing Vitiello’s nomination.

The Democratic chairs are seeking from Homeland Security “all communications related to the departure, or possibility of departure” of Nielsen, Alles, Grady and Vitiello from January 2019 to the present. They’re also asking for such documents as they relate to U.S. Citizenship and Immigration Services Director L. Francis Cissna and general counsel John Mitnick, both of whom are rumored to be on the chopping block in the administration.

The Democrats are also seeking all communications between McAleenan and Miller. They asked that Homeland Security provide the documents by May 9.

DHS press secretary Tyler Houlton told The Hill that “under no circumstances is the safety or security of the United States at risk during a leadership transition at DHS."

"The Department has successfully overseen the transition between a half-dozen Secretaries over the course of our 16-year history. At this time, the overwhelming majority of the senior positions at DHS are filled with either Senate confirmed or Presidentially Appointed officials or someone pending a current Senate confirmation," Houlton said. "Under the Acting Secretary’s leadership, every employee at the Department is committed to our mission of safeguarding the American people.”

Representatives for the White House did not immediately return requests for comment on Thursday.

The investigation is one of several House Democrats have opened with their majority powers in the House. The White House has signaled that it is intent on stonewalling the Democrat-led probes.

In a letter sent Wednesday, White House counsel Pat Cipollone told Cummings that the White House would “respectfully decline” his panel’s invitation for Miller to testify about Trump’s immigration policies, arguing that Miller is immune from the congressional testimony process based on past precedent.

Cummings, Nadler and Thompson — who lead the House Oversight, Judiciary and Homeland Security committees, respectively — have signaled they are intent on pressing forward aggressively with their probes into Trump and his administration.

--Updated April 25 at 8:36 p.m. to include DHS comment