WASHINGTON (Reuters) - The chairmen of three U.S. House committees sought documents on Thursday related to recent Trump administration firings of top Department of Homeland Security officials, escalating tensions between congressional Democrats and the White House over immigration policy.

FILE PHOTO: White House adviser Stephen Miller walks across the tarmac to board Air Force One as he departs Washington with U.S. President Donald Trump for travel to Grand Rapids, Michigan from Joint Base Andrews, Maryland, U.S., March 28, 2019. REUTERS/Joshua Roberts/File Photo

U.S. Representatives Elijah Cummings, Bennie Thompson and Jerrold Nadler, Democrats who head the House of Representatives Oversight, Homeland Security and Judiciary committees, said in a statement they were “concerned that the president may have removed DHS officials because they refused his demands to violate federal immigration law and judicial orders.”

The chairmen said they were particularly interested in documents relating to recent moves by President Donald Trump and Stephen Miller, a former congressional aide who has become a top presidential adviser on immigration, to purge DHS of “senior leaders” who “reportedly refused orders to violate the law.”

The committee leaders’ move followed a White House declaration that it was refusing a request from the Oversight Committee for Miller to testify before the panel.

In a letter to the committee on Wednesday, the White House said Miller would not testify about administration immigration initiatives, including Trump’s policy of separating migrant children from their parents and his threat to send immigrants in the country illegally to so-called sanctuary cities.

“In accordance with longstanding precedent, we respectfully decline the invitation to make Mr. Miller available for testimony before the committee,” the White House counsel said in the letter, which was provided to Reuters on Thursday.

The Republican president is pushing back against legal requests from Democratic-led House committees, which are conducting wide-ranging investigations of Trump and his administration, including his tax returns, White House security clearances and possible obstruction of justice by Trump.

DHS DEPARTURES

In their Thursday letter, the Democratic committee chairmen noted that DHS recently announced the departures of Homeland Security Secretary Kirstjen Nielsen, the department’s undersecretary for management, the director of the U.S. Secret Service and the acting director of the Immigration and Customs Enforcement (ICE) agency.

Cummings, whose committee has launched multiple investigations into Trump administration and White House policies, accused Trump on Wednesday of an “unprecedented, and growing pattern of obstruction” after he ordered federal employees not to comply with congressional investigations.

Cummings on April 17 invited Miller to testify voluntarily about why the administration decided to separate immigrant children from their parents at the border.

Cummings also called for an explanation of “transferring asylum seekers to sanctuary cities as a form of illegal retribution against your political adversaries, and firing top administration officials who refuse orders to violate the law.”

Trump has said he is considering sending immigrants in the country illegally to jurisdictions that have adopted some form of “sanctuary city” policies in which they refuse to use their resources to help federal agents enforce deportations.

Miller, a former Senate aide, helped shape some of Trump’s most contentious immigration policies, from a ban on Muslim immigrants proposed shortly after Trump took office in 2017 to the child separation policy, both of which were rejected by courts.

The oversight panel could move to subpoena Miller, but the White House could invoke executive privilege to protect his discussions with Trump.