President Donald Trump said Thursday he had not read former Blackwater CEO Erik Prince’s mass deportations proposal, but he “wouldn’t be opposed” to using private forces to help carry out his vow to remove millions of undocumented immigrants.

The move, which could expose unregistered migrants to criminal prosecution, represents a drastic escalation of the administration’s efforts to push millions of immigrants to leave on their own.

But lawyers and immigrant advocates warned that the administration’s plan was on shaky legal ground and that some immigrants may be afraid to comply.