The hijackers in the September 11 attacks were 19 men affiliated with al-Qaeda, and 15 of the 19 were citizens of Saudi Arabia. Others were from Egypt and the UAE. The hijackers were organized into four teams, each led by a pilot-trained hijacker with four "muscle hijackers" who were trained to help subdue the pilots, passengers, and crew.
The first hijackers to arrive in the United States were Khalid al-Mihdhar and Nawaf al-Hazmi, who settled in the San Diego area in January 2000. They were followed by three hijacker-pilots, Mohamed Atta, Marwan al-Shehhi, and Ziad Jarrah early in the summer of 2000 to undertake flight training in south Florida. The fourth hijacker-pilot, Hani Hanjour, arrived in San Diego in December 2000. The rest of the "muscle hijackers" arrived in the spring and early summer of 2001, just months before the September 11 attacks.