The "unknown" guys who threaten the plane are Tom Bowen and Zack White. At the very beginning of the movie, Bill is talking to a man at the airport. The man is smoking. That man is Tom, and later on, it turns out he is one of the hijackers. The other hijacker is Zack, who is sitting in the row behind Bill. He was originally supposed to sit next to Bill, but Jen traded seats with him. Thus, it was less obvious that Zack was a hijacker. As expected, the plane lands safely, happy ending.