The chairs of three House committees sent a letter to the Department of Homeland Security (DHS) Thursday requesting documents related to the abrupt departure of several top officials from the agency this month, including former Secretary Kirstjen Nielsen.

The Democratic committee chairs are specifically investigating the role that President Trump and his senior adviser, Stephen Miller, played in forcing these officials out in order to enact more hard-line immigration policies. Reps. Elijah Cummings of the Judiciary Committee, Jerry Nadler of the Judiciary Committee and Bennie Thompson of the Homeland Security Committee wrote that they are "deeply concerned that the firing and forced resignation of these officials puts the security of the American people at risk."

The administration announced that Nielsen, Secret Service Director Randolph "Tex" Alles, DHS Undersecretary for Management Claire Grady, and Acting Director of Immigration and Customs Enforcement Ronald Vitiello were departing from their senior leaderships positions this month -- a move that some Democrats saw as a purge. Miller was the driving force behind these resignations.

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CBS News Chief Washington Correspondent Major Garrett reported earlier this month that Mr. Trump has privately fumed for months about the increased flow of undocumented migrants arriving at the southern border, and congressional opposition to tightening asylum laws or speeding deportation. Miller won a crucial internal battle in late March when the president gave him control over immigration policy and Homeland Security personnel.

Mr. Trump's senior adviser and son-in-law, Jared Kushner, had been lobbying the president to work with Congress in pursuit of new laws. But the president refused and gave new power to Miller, who is looking to revive the family separation policy and close ports of entry.

In the process, the president is creating turbulence inside the country's third-largest federal agency, one with crucial obligations ranging from combating terrorism to protecting the president and the nation's ports.

Also on Thursday, the White House refused to allow Miller to testify before the House Oversight Committee. Cummings requested last week that Miller appear before the committee on May 1 to discuss immigration.

White House Counsel Pat Cipollone sent a letter to Cummings Thursday informing the committee that Miller would not testify, but that the administration was willing to make other officials available to the committee. The letter was first reported by the Washington Post.