There were supposed to be 5 hijackers on each plane. That would be 20. However, there were only 19. Moussaoui was the 20th hijacker. He was taken into custody by American authorities a month before the hijacking and they had reason to suspect he was part of a plot to hijack an airliner. He probably knew about the whole operation but they could never get anything out of him.
If the FBI could have questioned him the way the Israelis would have questioned him, there would be 7000 people alive today.