(CNN) Three powerful Democratic congressmen are seeking more information about the recent departures of multiple senior Department of Homeland Security officials, including Secretary Kirstjen Nielsen, as President Donald Trump threatened to shut down parts of the US-Mexico border.

The members of Congress wrote acting Homeland Security Secretary Kevin McAleenan on Thursday requesting all communications related to the departures of Nielsen, her deputy Claire Grady, Secret Service Director Randolph "Tex" Alles, acting Immigration and Customs Enforcement chief Ronald Vitiello and the possibility of others leaving. They requested the documents by May 9.

"We are deeply concerned that the firing and forced resignation of these officials puts the security of the American people at risk," wrote the chairmen.

Reps. Elijah Cummings of Maryland, Jerry Nadler of New York and Bennie Thompson of Mississippi, the heads of the Oversight, Judiciary and Homeland Security committees respectively, wrote that they were concerned White House aide Stephen Miller is taking greater control of the administration's immigration and border security agenda.

Department of Homeland Security press secretary Tyler Houlton said the safety of Americans is not at risk during leadership changes.

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