That simple fact, published on the Federal Aviation Administration's Web site, likely played a role in at least three of the hijackings that ended in disaster Tuesday.

Barbara Olson, wife of U.S. Solicitor General Theodore Olson, was on the American Airlines flight that crashed into the Pentagon. She told her husband during a cell phone call from the plane that the hijackers were using knife-like instruments.

Federal regulations on such weaponry state: ``FAA guidelines allow knives with blades up to 4 inches. However, state and local laws may restrict the carriage of smaller knives in a public airport. We recommend that you contact the airline to determine any additional restrictions it might apply.''

Two AP journalists who travel frequently said they themselves have dropped Swiss Army knives into plastic containers as they walked through airport metal detectors, only to pick them up afterward, and carry them aboard.

American Airlines, citing the government's ongoing investigation into four jetliner crashes, said it would be ``inappropriate'' to comment on its policies.

The airline's Web site suggests passengers check with someone before packing weapons such as ``firearms, ammunition, gunpowder, mace, tear-gas, or pepperspray.'' The message does not specifically mention knives, but says items classified as dangerous goods ``may be restricted for transport by air.''

A spokeswoman for Air Canada said her company does not allow knives as carry-on property, and a former official with British Airways said the same.

British Airways often collects knives from passengers who attempt to bring them on board. The crew then stores the blades until the flight lands. The former British official, who now works as a travel consultant and spoke on condition of anonymity, said he learned during 18 years in the airline industry that carriers based abroad are usually more stringent about security than their domestic competitors.

While firearms are not permitted in passenger areas, some airlines do allow them to be stored in the baggage compartment.

Delta, for example, allows customers one item of ``shooting equipment'' as part of the passenger's free checked baggage.

The list of allowable weapons includes rifles, pistols, and up to 11 pounds of ammunition. An excess baggage charge is levied on any passenger bringing more than one of these items.

Customers checking a weapon must present it, unloaded, to a Delta employee, who is supposed to make sure it is in a locked, hard-sided, crushproof container to be stored below.