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rorists (some 60,000 before 9/11) until he heard it mentioned during the
Commission's January 26, 2004, public hearing.The FAA had access to some
TIPOFF data, but apparently found it too difficult to use.

The second part of prescreening called on the air carriers to implement an

FAA-approved computerized algorithm (known as CAPPS, for Computer
Assisted Passenger Prescreening System) designed to identify passengers whose
profile suggested they might pose more than a minimal risk to aircraft.
Although the algorithm included hijacker profile data, at that time only pas-
sengers checking bags were eligible to be selected by CAPPS for additional
scrutiny. Selection entailed only having one's checked baggage screened for
explosives or held off the airplane until one had boarded. Primarily because of
concern regarding potential discrimination and the impact on passenger
throughput, "selectees" were no longer required to undergo extraordinary
screening of their carry-on baggage as had been the case before the system was
computerized in 1997.

This policy change also reflected the perception that

nonsuicide sabotage was the primary threat to civil aviation.

Checkpoint screening was considered the most important and obvious layer

of security. Walk-through metal detectors and X-ray machines operated by
trained screeners were employed to stop prohibited items. Numerous govern-
ment reports indicated that checkpoints performed poorly, often failing to
detect even obvious FAA test items. Many deadly and dangerous items did not
set off metal detectors, or were hard to distinguish in an X-ray machine from
innocent everyday items.

While FAA rules did not expressly prohibit knives with blades under 4

inches long, the airlines' checkpoint operations guide (which was developed in
cooperation with the FAA), explicitly permitted them.The FAA's basis for this
policy was (1) the agency did not consider such items to be menacing, (2) most
local laws did not prohibit individuals from carrying such knives, and (3) such
knives would have been difficult to detect unless the sensitivity of metal detec-
tors had been greatly increased. A proposal to ban knives altogether in 1993
had been rejected because small cutting implements were difficult to detect and
the number of innocent "alarms" would have increased significantly, exacer-
bating congestion problems at checkpoints.

Several years prior to 9/11, an FAA requirement for screeners to conduct

"continuous" and "random" hand searches of carry-on luggage at checkpoints
had been replaced by explosive trace detection or had simply become ignored
by the air carriers. Therefore, secondary screening of individuals and their
carry-on bags to identify weapons (other than bombs) was nonexistent, except
for passengers who triggered the metal detectors. Even when small knives were
detected by secondary screening, they were usually returned to the traveler.
Reportedly, the 9/11 hijackers were instructed to use items that would be
undetectable by airport checkpoints.

In the pre-9/11 security system, the air carriers played a major role. As the

THE 9/11 COMMISSION REPORT

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