Three House Democratic chairmen on Monday launched a probe into reports that the White House pressured immigration enforcement officials to release migrants into the districts of political adversaries.

In a letter to Acting Chief of Staff Mick Mulvaney and Acting Secretary of Homeland Security Kevin K. McAleenan, the head of the Judiciary, Oversight and Homeland panels demanded all emails and communications on the policy.

“These reports are alarming,” wrote Judiciary Chairman Jerrold Nadler (D-N.Y.), Oversight Chairman Elijah E. Cummings (D-Md.) and Homeland Security Bennie G. Thompson (D-Miss.). “Not only does the administration lack the legal authority to transfer detainees in this manner, it is shocking that the president and senior administration officials are even considering manipulating release decisions for purely political reasons.”

The demand for documents comes as Trump embraces the idea of busing migrants to sanctuary cities, arguing that Democrats should applaud the idea — or make a deal with him on immigration. While White House press secretary Sarah Sanders initially tried to downplay the proposal, on Sunday she said it was still on the table.

“Certainly, we’re looking at all options,” Sanders said during an appearance on “Fox News Sunday.” She said the proposal is not an “ideal solution,” but that if Democrats refuse to negotiate with Trump on border security, the White House is prepared to “put some of those people into their communities” and see how the Democrats react.

The proposal would put Trump at odds with not just Democrats but his own administration. DHS officials have determined that there is no legal basis for punishing political opponents by releasing migrants into their districts and have express worries about the optics. 

Additionally, top U.S. Immigration and Customs Enforcement officials have said the policy could create an undue burden on their agency, sucking up resources at time when there’s a crisis at the border. Others worry it would actually give migrants an incentive to come to the U.S.

Congressional Republicans, meanwhile, have said little, fearful of going up against a president popular with their base.

In addition to the document requests, House Democrats are eyeing White House official Stephen Miller as a top target for an investigation. Miller, while not a confirmed Cabinet head, has reportedly influenced a number of Trump’s more controversial immigration policies. Democrats say he should, therefore, come answer questions.