Sometimes dubbed the "20th hijacker" in the 9/11 attacks, Zacarias Moussaoui may have been little more than a terrorist groupie. Moussaoui underwent flight training in the U.S. but flunked out so spectacularly that his flight instructor contacted the FBI, unsure why a student so incompetent was seeking training in the first place. Moussaoui was arrested in August 2001 on immigration charges, and investigators later found evidence linking him to international terrorism.
After 9/11, authorities believed Moussaoui planned to join the roster of hijackers, and Moussaoui himself said he and Richard Reid had planned to hijack a fifth plane to crash into the White House. But captured 9/11 mastermind Khalid Sheikh Mohammed told his interrogators that Moussaoui was never involved in the plot; there's little evidence that Moussaoui and Reid ever met; and al-Qaeda reportedly had concerns about his reliability. Moussaoui was nonetheless found guilty on terrorism charges in 2006, but during sentencing, three jurors wrote a statement saying they believed he had "limited knowledge" of the pending 9/11 attacks. For that reason, Moussaoui was narrowly spared the death penalty and is serving a life sentence without the possibility of parole. To date, he's the only person sentenced in connection with the 9/11 attacks.