Back in the 1990s both the FBI and the Federal Aviation Authority became concerned about the possibility of terrorist attacks on US airlines. The chief concern of the security officials was that someone might buy a ticket, check luggage containing a time-bomb, and then not board the flight (allowing some anxious college student traveling stand-by to heave a sigh of relief and rush to take the seat suddenly made available). To address this problem, a Computer-Assisted-Passenger-Pre-Screening (CAPPS) system was created. The system cross-checked names of passengers against security “watch lists.” If a passenger name appeared on a “watch list,” the passenger’s checked luggage was held off the plane until the passenger had actually boarded the plane and could be screened for explosives. No supplementary examination of the passenger took place.

CAPPS formed the chief line of defense for the airlines on 11 September 2001.

Airlines and government security officials did not anticipate either suicide bombings or hijackers turning the planes themselves into cruise missiles. Was there any basis for anticipating such acts?

A dozen airliners were hijacked in the 1960s; 42 were hijacked in the 1970s; 24 were hijacked in the 1980s; 16 had been hijacked in the 1990s; and 6 were hijacked in 2000 or early 2001. It would have been easy to assume that this problem was well on its way to being controlled. Occasionally–once in 1987, once in 1988, and twice in 1994–there had been failed attempts to seize an airliner to use as a cruise missile against a building. In 1991 Shamil Besaev, a leader of the Chechen opposition to Russia, had hijacked a jet.

In 1983 Hezbollah suicide bombers had attacked the American embassy, and the U.S. Marine and French barracks in Beirut. About fifty other suicide attacks followed.

Between 1987 and 2000 the Tamil Tigers carried out many perhaps as many as 175 suicide bombings.

Between July 1989 and December 2000 Hamas and Palestinian Jihad had carried out 28 suicide attacks directed against Israel. The attacks had killed 170 people. Between January and August 2001, Hamas and Palestinian Jihad had conducted 26 suicide attacks directed against Israel. The attacks had killed 52 people. So, a sudden spike in suicide attacks.

In Summer 2000, Chechen suicide attacks on Russian troops took place in the North Caucasus region. These were ordered by Shamil Basaev.

Most pertinently, Al Qaeda had used suicide bombers to attack the U.S. embassies in Kenya and Tanzania in August 1998 and the U.S.S. Cole in October 2000.

That amounts to a lot of disparate pieces of the jig-saw puzzle to put together.

Boston: American 11 and United 175. In Portland, Maine, boarding his connecting flight to Logan Airport, Mohamed Atta had been identified by CAPPS as subject to special security precautions: his checked luggage was held aside until it was confirmed that he had boarded the plane. (p. 4.) At Logan, three other members of his team were identified by CAPPS and the same restriction on loading luggage was applied. (p. 5.)

Washington Dulles: American 77. Three members of the hijacking team were selected by CAPPS; two others were picked out by a suspicious counter agent. All five had their checked luggage held off the plane until it was confirmed that they had boarded the plane. (pp. 5-6.)

Newark: United 93. One of the hijacking team members was selected by CAPPS. His checked bag was checked for explosives, then loaded on the plane when none were detected. (p. 7.)