Rep. Jerrold Nadler, D-N.Y., on Sunday said White House aide Stephen Miller, “who seems to be the boss of everybody on immigration,” should appear in front of Congress and try to explain recent developments in policy, including the idea to send migrants from the border to sanctuary cities.

Nadler told CNN that he learned from “whistle-blowers” that Miller was behind the idea to place the illegal immigrants in these cities.

White House Press Secretary Sarah Sanders confirmed to "Fox News Sunday" that President Trump's prospective plan to send illegal immigrants to sanctuary cities is undergoing a "complete and thorough review," days after Democrats who have fought to protect illegal immigrants from federal authorities characterized the possible move as a dangerous stunt.

Trump has grown increasingly frustrated over the situation at the border, where tens of thousands of immigrant families are crossing each month, many to claim asylum. His administration has attempted several efforts to stop the flow and he recently shook up the top ranks of the Department of Homeland Security.

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The idea to ship immigrants to Democratic strongholds was considered twice in recent months, but the White House and Department of Homeland Security said the plan had been rejected.

Fox News' Gregg Re and the Associated Press contributed to this report