That's part of the agreement - they are to be returned to Mexico while awaiting US action.
See this link:
http://www.freerepublic.com/focus/f-news/3755325/posts?page=64#64
Every Republican should become a “bounty hunter” a payment requiring only the IDing of an illegal alien, and resulting in a successful apprehension.
Trump could federalize the NG in certain states, and have them “detain” the criminal invaders for a quick review by ICE and then transit them to the border.
It would be “fine pickins” in the so-called sanctuary cities. Many more would likely self-deport.