FAA warned of disguised weapons but 9/11 hijackers used permitted knives

On August 16, 2001, the Federal Aviation Administration (FAA) issued a circular titled "Disguised Weapons" (IN-2001-12) to airlines, warning that terrorists might use everyday objects modified into weapons to hijack an airplane. This warning, however, had no real impact on airport security because the 9/11 hijackers used only knives and razors, which were permissible to carry at that time. The knives and razors were essential for the hijackings, as flight attendants and passengers who resisted in First Class were stabbed and cut when the plane crews were overwhelmed, as evidenced by phone calls from the doomed planes.

Khalid Sheikh Mohammed (KSM), the mastermind of the 9/11 attacks, began experimenting with bringing knives and razor blades (commonly called box cutters) aboard flights in December 1999. His close friend Walid bin Attash (known as "Khallad") carried a razor knife on a flight from Pakistan to Malaysia on a casing flight. Khallad subsequently tested airline security measures on U.S. carrier flights to Malaysia, Bangkok, and Hong Kong. In January 2000, while escorting Khalid al-Mihdhar and Nawaf al-Hazmi (the San Diego duo) to Malaysia, he again brought a knife onboard. He later briefed KSM and Mohammed Atta about his experiences.

Atta, as he returned to the United States from Zurich on July 8, 2001 (after meeting with Ramzi Bin al-Shibh in Spain), purchased two Victorinox Swiss Army knives at the Zurich airport, bringing them with him onto the plane and into the United States. The FBI, through a meticulous review of financial records, determined after 9/11 that terrorists aboard each flight purchased at least seven knives. On August 13, 2001, Marwan al-Shehhi (pilot of UA Flight 175 that hit the South Tower of the World Trade Center) purchased two short-bladed knives, a Cliphanger Viper and an Imperial Tradesman Dual Edge, at Sports Authority in Boynton Beach, Florida. Fayez Banihammad and Hamza al Ghamdi ("muscle" men aboard the same flight) then bought a Stanley two-piece snap knife set and a Leatherman Wave multi-tool at Wal-Mart in Boynton Beach. On August 27, 2001, Nawaf al-Hazmi (a muscle man on AA Flight 77 that hit the Pentagon) purchased a Leatherman Wave Multi-tool from Target in Laurel, Maryland. Financial records do not confirm weapons purchases by any of the hijackers on UA Flight 93 that crashed in Pennsylvania, but the FBI recovered 14 knives or portions of knives, including a box cutter, from the wreckage. None of the blades or knife housings recovered indicated a blade length longer than 3.5 inches.

Ramzi Bin al-Shibh, captured after 9/11, told interrogators that during his meeting with Mohammed Atta in Spain, they discussed how the hijackings would be executed. He said Atta, al-Shehhi, and Ziad Jarrah (pilot of the Pennsylvania plane) had encountered no problems carrying box cutters on cross-country domestic test flights. This information highlights the security vulnerabilities that existed before 9/11, as the FAA's warning did not lead to changes in the rules that allowed such weapons to be carried onto planes.