Nearly half of the Sept. 11 hijackers were allowed to board their flights even after they were singled out by airline security, a government official said.

The fact that the nine hijackers still got through prompted some to wonder how the procedures meant to protect air passengers' lives could have failed so badly.

"Clearly the system failed even worse than is generally known," said Paul Hudson, who lost his daughter in the bombing of Pan Am 103. "There should be no reason to place high confidence in it again."

Hudson now runs the Aviation Consumer Action Project, an advocacy group affiliated with consumer advocate Ralph Nader.

A government official confirmed that six hijackers were flagged by a computerized airline passenger profiling system; two others were singled out because of questions with their identification; and a third was marked because he was traveling with one of the passengers with questionable ID, said the official, speaking on condition of anonymity.

The planes they boarded would later smash into and topple the World Trade Center, demolish a section of the Pentagon and crash into a field in rural Pennsylvania in the worst act of terrorism in history.

The searches were first reported by The Washington Post.

Under the security procedures in place at the time, officials would inspect the flagged passengers' checked luggage for explosives, either by hand or by machine. They would have already screened the passengers and their carry-on bags for weapons.

The hijackers used box cutters and knives to take over the airplanes, but those items were allowed to be carried on board before the terrorist attacks.

The pre-Sept. 11 procedures were put in place to prevent airline bombings, as was the case with Pan Am Flight 103 in December 1988. Airline crews were taught to cooperate with hijackers as the best way to ensure that a plane lands safely. Federal aviation officials did not anticipate suicide terrorists turning commercial airliners into weapons.

The computerized profiling system has since been modified to single out more passengers for extra scrutiny.

In addition, airlines are now checking the names of passengers against government lists of potential terrorists, sometimes with software offering alternative spellings of Arabic names to prevent people from evading detection by using different translations.

It has already been reported that airline security officials did not know on Sept. 11 that two of the hijackers were on an FBI watch list of potential terrorists.

A group of relatives of the victims of Sept. 11 has called for an investigation of airline security that day, a request echoed by Hudson.

"This showed why we need a full independent investigation of what happened" on Sept. 11, Hudson said. "We haven't heard the whole story."

The Associated Press contributed to this report.