No greater witness exists in the events of “Elephant” than the focused, almost predatory movie camera. It begins by plodding rather aimlessly through school hallways, occasionally stalking lone figures whose movements are indicative of grinding routine. Their faces come into view during stationary pauses, but most of their dialogue, succinct and shallow, is often muffled behind an ominous soundtrack. Here there are the idle paces of teenagers with unknown agendas, and facial expressions that seem evaporated of enthusiasm: for all of them, this is apparently nothing more than a normal school day. But then a brewing storm of tragedy reveals itself almost accidentally, when one of the figures steps out of the building, catches sight of two teenage classmates dressed in camouflage and notices they are carrying duffle bags and snarls of resentment. “Get out and don’t come back,” one of them warns him. “Some heavy shit’s going down.” Their proclamation casts a shadow on everything we observe, and suddenly the camera is no longer just a device, but a powerless onlooker documenting a series of unrelated lives before they are all altered by something devastating and unforeseen.