These cases present a narrow question with several related issues. May the Attorney General, as the executive head of the Immigration and Naturalization Service,[1] after taking into custody active alien Communists on warrants,[2] charging either membership in a group that advocates *527 the overthrow by force of this Government[3] or inclusion in any prohibited classes of aliens,[4] continue them in custody without bail, at his discretion pending determination as to their deportability, under § 23 of the *528 Internal Security Act?[5] Differing views of the Courts of Appeals led us to grant certiorari. 342 U. S. 807, 810.