The 9/11 commission:
In the fall of 1999, training for the attacks began. Hazmi, Mihdhar, Khallad, and Abu Bara participated in an elite training course at the Mes Aynak camp in Afghanistan. Afterward, KSM taught three of these operatives basic English words and phrases and showed them how to read a phone book, make travel reservations, use the Internet, and encode communications. They also used flight simulator computer games and analyzed airline schedules to figure out flights that would be in the air at the same time. Kuala Lumpur
Following the training, all four operatives for the operation traveled to Kuala Lumpur, Malaysia. Khallad and Abu Bara were directed to study airport security and conduct surveillance on U.S. carriers, and Hazmi and Mihdhar were to switch passports in Kuala Lumpur before going on to the United States. Khalladwho traveled to Kuala Lumpur ahead of Hazmi and Mihdharattended a prosthesis clinic in Kuala Lumpur. He then flew to Hong Kong aboard a U.S. airliner and was able to carry a box cutter, concealed in his toiletries bag, onto the flight. He returned to Kuala Lumpur, where Hazmi and Mihdhar arrived during the first week in January 2000. The al Qaeda operatives were hosted in Kuala Lumpur by Jemaah Islamiah members Hambali and Yazid Sufaat, among others. When Khallad headed next to a meeting in Bangkok, Hazmi and Mihdhar decided to join him to enhance their cover as tourists.