I think Brett Partridge is RJ. Using his forensic skills he can fake his own death, then when Lisbon looked away, he grabbed her feet. The door  was opened by an assistant.

In an interview Bruno Heller said it wouldn't be like the story 10 Little Indians, with an RJ suspect dying every week, but in the book of that story, the main killer fakes his own death right at the start, then goes and kills others, so Partridge may be the first to 'die', but like in the book, he faked it.