US President Donald Trump speaks during a roundtable on immigration and border security at the US Border Patrol Calexico Station in Calexico, California, April 5, 2019.

Top Democrats in the House of Representatives on Monday sent a letter to the White House and the Department of Homeland Security demanding documents related to the Trump administration's plan to release detained immigrants in cities that shelter undocumented immigrants.

The plan, which was rejected twice in the past year, was confirmed to be still in the works by President Donald Trump last week after its existence was first reported by The Washington Post.

In a post Monday on Twitter, the president said the plan was moving forward, writing in a tweet that "Illegal Immigrants who can no longer be legally held (Congress must fix the laws and loopholes) will be, subject to Homeland Security, given to Sanctuary Cities and States!"

Trump tweet

The announcement that the plan was still under consideration marked a dramatic reversal from the stance other administration officials had taken.

One DHS official told reporters Thursday that the plan "was a suggestion that was floated and rejected, which ended any further discussion." And a former DHS official told NBC News that the White House dropped the plan after it was deemed "so illegal."

Now Democrats are seeking emails between the White House and DHS officials, as well as documents "discussing policy or legal justifications or implications" for the plan, according to the letter, which is signed by the chairs of the Judiciary, Oversight and Homeland Security committees.

"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," wrote Reps. Jerry Nadler, D-N.Y., Elijah Cummings, D-Md., and Bennie Thompson, D-Miss.

The representatives are seeking documents between November 2018 and April 15, 2019, and gave the White House until May 3 to make them available. The letter was sent to Mick Mulvaney, acting chief of staff, and Acting DHS Secretary Kevin McAleenan.

The report in the Post noted that the plan was dismissed twice in the last six months after legal officials rejected it and also raised concerns that it could heighten public relations problems.

It said the plan sought to penalize Democrats whose congressional districts were located in so-called sanctuary cities, including House Speaker Nancy Pelosi, who represents parts of San Francisco. Trump has said that Democrats should be "very happy" about the plan.

The White House did not immediately provide comment.