My wife and I watched this last year, so it's 2014 or older, but probably from the 21st century. It was a live-action English-language film, probably from the United States. A group of people wake up in a largely featureless room (there's just the room, which I think had white walls and a high ceiling, an attached small bathroom (with sink, western toilet, and paper towel dispenser), and I think a corridor with a door at the end (which they are forbidden to go down, with one person killed for ignoring that rule)). An announcement is made over the loudspeakers that they must follow certain rules (not fiddling with their explosive collars (or maybe the collars just delivered a fatal electric shock), not attempting to escape, etc) and that there are criminals in their midst (if I recall correctly, at the least, a serial killer and a pedophile are named among the people there) and told that the killer will start eliminating them one by one. There were clues scattered around the room about who the killer is. I think I remember there being one in the paper towel dispenser and another in the toilet paper in the bathroom. The people in the room included an olive-skinned priest (I want to say he was either Indian or South-American), a school official (doughy looking and red-haired), and a Hispanic gentleman who was either a police officer or a criminal (I honestly don't remember if they every clarified which he was, I but I think people accused him of being both at one point or another).
They also had some items in the room. There was a gun with a single bullet (which I remember being put in a lock-box on one side of the room, later retrieved by one of the characters), a vial of poison (probably supposed to be cyanide based on the "foaming at the mouth" effect it had), and a hammer and a screwdriver. Among the "clues" were two halves of a key, and at some point, the prisoners start trying that key on their collar in a blind-spot in the room. Needless to say, multiple people die in the course of the scenario, some by the killer and some by paranoid members of the room. There's also the one requisite person killed at the very beginning to prove that the collars are real. Oh, and I vaguely remember them trying to either dig through a brick wall or use an improvised explosive on it, but I don't remember what happens there.
The prisoners are told that they're under constant surveillance, and I think I remember at least one shot ostensibly from the cameras watching the prisoners complete with the lowered image quality. Another slightly odd thing, there was one incidence of Hollywood Darkness where we as the audience can see despite the scene supposedly being entirely unlit, but they used red instead of blue for the lighting.
Ultimately, it turns out that the attractive blond female who was the main protagonist was one of two plants in the room, and was the killer. After everyone else was dead, she left the room, did some paperwork on the people in the room, and then went through another door to become the plant for another group, which I remember included a guy in a wheelchair. I think it was implied that this was all training for deep-cover intelligence agents.
In terms of people killed:
I remember that one person was slashed up in a sudden blackout, the priest was beaten to death while tied up, the school-lady was indicated to be the pedophile and died from the poison (I think, self-administered), and someone was shot with the single bullet.
I will comb through my library reading list to see if I can find it, but I figured I would also appeal to the community. If it helps, the 2006 film, Unknown might have been in the pre-movie trailers, or vice versa. If so, that might imply that the film is by Eleven Eleven Films. The Killing Room looks very similar in style, but the details (and the ending) don't match up. Nine Dead matches up on a number of plot points (pedophile, priest, petty criminal, law officer, people kidnapped and being killed one by one) but again, the details don't match up.
Movies I know it is not:
- Any of the Saw franchise (I've seen all but the 7th one, and it lacks the same sort of feeling).