Three powerful House committees opened an investigation into the motives behind President Trump’s recent removal of senior leadership at the Homeland Security Department.

In a letter sent Thursday to the agency’s acting secretary Kevin McAleenan, the panels’ chairmen asked for all communications related to the departure of several individuals, including former homeland secretary Kirstjen Nielsen, who resigned on April 7.

They also requested all communications with White House senior adviser Stephen Miller about department leadership changes.

Chairmen Elijah E. Cummings (Md.) of Oversight, Jerrold Nadler (N.Y.) of Judiciary and Bennie G. Thompson (Miss.) of Homeland Security wrote that they are “concerned that the President may have removed DHS officials because they refused his demands to violate federal immigration law and judicial orders.”

They also said they were concerned that the exits of top Homeland staff empowered Miller to handle the Trump administration’s immigration policy.

Last week, Cummings sent a letter asking Miller to testify about his role in driving the White House’s most controversial immigration ideas, both implemented and proposed. The White House responded Wednesday night that Miller would not be made available to speak to Congress.

Homeland Security officials have told The Washington Post that Miller was the architect of the never-actualized plan to bus migrants who cross the border illegally to sanctuary cities represented by Trump’s political foes. He also reportedly was behind the zero-tolerance policy, which led to the separation of migrant children from their parents once they crossed the border.

Cummings, Nadler and Thompson allege that Miller’s reported role in the ouster of Homeland top brass is “part of a pattern of Mr. Miller pressuring administration officials” to take extreme immigration positions.