They suspended his Miranda rights under a national security exception, and given he is a suspect in a domestic terrorist attack I'm not sure the matter of his citizenship should give him advantages that a foreign combatant is denied.
He engaged in foreign combatant activity against civilians on our soil.
The first part is fine. I understand claiming the exemption because you don't know what else they planned, if they were alone, or whether they planted more devices. But you don't just forfeit a U.S. citizen's constitutional rights for actions on American soil without due process no matter how severe. If they were foreign nationals that would be a different story for me, but he's a U.S. citizen whether they like it or not. If he want's to revoke his citizenship then have at it, but until then he has his rights.