Three House committee chairmen are demanding to see internal White House and Homeland Security communications related to President Donald Trump’s proposal to release migrants apprehended at the southern border into so-called sanctuary cities across the United States.

“These reports are alarming,” wrote Judiciary’s Jerrold Nadler, Oversight and Reform’s Elijah Cummings, and Homeland Security’s Bennie Thompson, in a letter Monday to acting White House Chief of Staff Mick Mulvaney and acting Homeland Security Secretary Kevin McAleenan.

The three Democrats wrote that they believe the administration lacks the legal authority to transfer detainees in such a manner. They issued a May 3 deadline.

The three pointed to multiple reports of plans to send migrants to largely Democratic municipalities that prevent their police from cooperating with federal immigration authorities. It isn’t clear how such a program would be carried out, including how the migrants would be kept from leaving those areas.

On Friday, Trump tweeted, “Due to the fact that Democrats are unwilling to change our very dangerous immigration laws, we are indeed, as reported, giving strong considerations to placing Illegal Immigrants in Sanctuary Cities only.” On Sunday, press secretary Sarah Sanders told ABC’s “This Week” that the administration is “doing a full and thorough and extensive review.”

In their letter, the Democrats said, “It is shocking that the president and senior administration officials are even considering manipulating release decisions for purely political reasons.”