As soon as you check your bags at an airport, airlines use a sophisticated computer system to identify if you're a security threat.

It's called CAPS — Computer Assisted Passenger Screening.

"We are looking for patterns of behavior that are unusual," said Jerry Kavar, a former member of the White House Commission on Aviation Safety and Security.

CAPS kicks in as soon as a ticket agent enters your name into a computer, that is — if you check a bag.

Aviation authorities say it's designed to flag suspicious passengers, including those who fly one-way or at the last minute.

"The automated passenger information system would cue the agent that is checking the person in that this is somebody who deserves special scrutiny," Kavar said. "The agent would not know why, but what that would mean is a very thorough search."

That's how the system is supposed to work. So if it was in place on Sept. 11, how did 19 hijackers get by unnoticed?

"If it is true that two of the hijackers were on the FBI Watch list and if CAPS was working the way the Commission intended, certainly those people would have been signaled out for additional scrutiny," Kavar said.

Sandra Highland of the National Research Committee says CAPS was designed only to prevent explosives from getting onto planes by alerting airline employees to the presence of suspicious bags.

"It was not designed to identify these sorts of people," she said. "I think the idea [that] people could take over the plane using boxcutters is not something we would imagine."

Kavar points out that CAPS may have failed for two reasons: most of the Sept. 11 hijackers didn't check any baggage and government agencies failed to share information.

"The CAPS system itself didn't fail," he said. "What has happened is the government failed."

In 1997, the White House commission recommended the FAA beef up CAPS, including the screening of all passengers and mandating that airlines use the computer system.

"You may never have a system which is perfect," Kavar said. "But you can certainly raise the bar a lot higher than it is now."

The FAA, which is responsible for implementing CAPS, says it is now working to implement the tougher security measures recommended years ago.