I apologize if the details I give are too vague, but they are all I can remember. In the movie a group of people are in some sort of abandoned building (possibly a mall, lab, etc.). As the movie progresses the people in the group begin to get picked off and murdered. The scene that I can remember the best is the remainder of the group finding some sort of control room to watch the surveillance tapes to see what happened to the now dead group members. On the video the group sees one of their current group members murdering the dead group member, but he has no knowledge of ever committing the murder. He claims that it wasn't him and that somebody else did it. I may be wrong when I say this, but I feel like there was something that could transform into a specific person in the group to cox a member alone and then kill them one by one. Please help me before I go insane!
As Walt mentioned in the comments, it sounds a lot like Book of Shadows: Blair Witch 2 (2000).
After the group go into the woods to investigate the original Blair Witch location strange and horrible things start happening and they have to go to the hospital:
While they're there, people start disappearing and acting strangely, and near the end of the film they check the security tapes for the building:
And at the end of the film the survivors are arrested and shown more videos: