In the dark, they come in, shielded from public view by a police perimeter and an inner line of uniformed guardians. The target is reserved for the Tier I forces: They’ve practiced hundreds of times, rappelling onto the roof, banging down doors, hop-scotching down corridors, sweeping rooms. When it’s all clear, the scientists are ushered in with their detection gear and their “render safe” machines and tools. The orders are crystal clear: Do everything and anything to get to the nuke before it explodes, take no chances, shoot to kill.