Two weeks after the September 11 attacks, the Federal Bureau of Investigation identified the hijackers[1] and connected them to al-Qaeda,[2] a global, decentralized terrorist network. In a number of video, audio, interview and printed statements, senior members of al-Qaeda have also asserted responsibility for organizing the September 11 attacks.

The primary responsibility falls upon the hijackers. As the media covered the 9/11 attacks unfolding, many quickly speculated that Osama bin Laden was behind the attacks.[20] On the day of the attacks, the National Security Agency intercepted communications that pointed to Osama bin Laden,[21] as did German intelligence agencies.[22] This helped rule out other immediate suspects, such as Croatian nationalists, who had bombed Grand Central Terminal on September 11, 1976.[23]

Mihdhar and Hazmi were also potential pilot hijackers, but did not do well in their initial pilot lessons in San Diego. Both were kept on as "muscle" hijackers, who would help overpower the passengers and crew, and allow the pilot hijackers to take control of the flights. In addition to Mihdhar and Hazmi, thirteen other muscle hijackers were selected in late 2000 or early 2001. All were from Saudi Arabia, with the exception of Fayez Banihammad, who was from the United Arab Emirates.