In the past couple of days I've seen a couple of horror movies that both end with a "big reveal" of a monster. Together they show there are good ways and bad ways to do this.
This film was actually remade in an American version recently called Quarantine. That movie told basically the same story as this one but changed a couple of details seemingly to play to the "conspiracy theory" crowd, such as suggesting the authorities are telling the world everyone has been removed from the house and is safe while they have really been left inside to die. The biggest change is in the final explanation for it all. There is no explicit "this is how it started" scene but at the end of both versions, the TV reporter and her cameraman, the last two survivors, ascend to the locked penthouse of the complex and discover a hidden laboratory. In the American version there are newspaper clippings on the wall about a doomsday cult and just vague hints about what has been happening. In the Spanish original, the clippings are about the demonic possession of a little girl and there is enough evidence mentioned from journals and tape recordings to enable the viewer to piece together the story and also make understand who the two monsters are that appear at the end. Like Let The Right One In and seemingly, its American remake, Let Me In, this shows that foreign horror films make a lot more sense these days than many of their American counterparts.