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Hijackers 'knew what they were doing'

(CNN) -- To pull off the coordinated aerial attack on the World Trade Center and Pentagon on Tuesday, the hijackers must have been extremely knowledgeable and capable aviators, a flight expert said.

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All four commercial jetliners seized in the air for the deadly assaults had just started transcontinental flights, ensuring that they carried the maximum amount of fuel.

One of the planes, United Airlines Flight 93, for example, could have carried as much as 200,000 pounds of the highly explosive fuel, according to one source, a veteran pilot.

By seizing four planes, diverting them from scheduled flight paths and managing to crash two into the twin towers of the World Trade Center and a third into the Pentagon, they must have had plenty of skill and training.

"They know what they were doing," said the source, a pilot with a major carrier for more than 30 years.

It was not known how the hijackers slipped through airport security checkpoints with their weapons.

How soon after departure the hijackers took over the planes remains unclear. But a handful of crew members and passengers managed to call authorities and loved ones on the ground, informing them that the planes had been commandeered, in some cases with deadly force, by assailants brandishing knives and who claimed to have bombs.

An open microphone on one of the flights recorded sounds of distress, according to reports.

There are no indications that any of the airline crews activated a four-digit code alerting ground controllers that a hijacking was in progress.

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