Breathing Room (2008)

Thrown naked into a desolate room with thirteen strangers, Tonya discovers that she is the final contestant in a deadly game. Restrained by lethal electronic collars, the players must utilize hints and tools from a box marked "pieces" to find both an exit and the reason for their abduction. One by one the players are eliminated as their "curfew" begins and the lights go out. With each dead body comes another clue, which they use to discover that one of them is the killer. The question is ... which one?