Who was the 20th hijacker?

Immediately after 9/11, investigators began piecing together the plot. In the flight manifests, they discovered a pattern: A team of five terrorists hijacked every plane that day, except for one. United Airlines Flight 93 had only four hijackers. Investigators believed one person was missing from that group.

Who was the 20th hijacker — if there even was one? And is there a 9/11 hijacker still on the loose?