Although airport security has become more sophisticated with explosive and metal detection machines, no level of technology can thwart determined terrorists, said Stephen Luckey, special security advisor to the Air Line Pilots Association.

What might, he said, is "a culture change," where passengers would allow the government to delve into their travel history, purchasing records and other personal information by way of a powerful, computerized, pre-screening program, initiated with a ticket purchase.

"People will have to invest a small amount of intrusion into their privacy to obtain the amount of security they desire," said Luckey, a pioneer in computerizing passenger screening and a retired Northwest Airlines captain. "It will require the P word: profiling."

The renewed push for profiling comes while passengers are being prohibited from placing liquids or gels in carry-on items, a ban that will be enforced indefinitely — but might be lifted within a few weeks, officials said.

"We don't believe the changes will be permanent, but they will remain in place until further notice," Christopher White, spokesman for the Transportation Security Administration, said today.

Luckey, of Kalispell, Mont., and other security experts say a high level of security isn't possible without profiling.

He noted people would not be identified based on how they look, but rather how they behave. For instance, a passenger who has traveled frequently to the Middle East might arouse suspicion, but that person may or may not be Middle Eastern, he said.

"You can be white, purple or green; it doesn't matter how you look," he said.

Luckey helped develop the earliest version of the pre-screening program currently in use. It crosschecks passengers' names against those on a few no-fly lists, a function he called "primitive."

The version now under development, called Secure Flight, would be overseen by the Transportation Security Administration and crosscheck names against more than 20 lists.

However, Secure Flight has been under fire since it was unveiled in 2003. The American Civil Liberties Union said it could be too intrusive into the average person's privacy.

More recently, in February, the Government Accountability Office warned the program might be ineffective because it has no clear objective.

"A general fear of terrorism should not force us to forgo our basic rights," Brandon Hensler, director of communications for the ACLU Florida office, said today.

Hensler said by drawing on many databases, the Secure Flight program could be "fraught with errors," endangering innocent people.

"We are absolutely for security," he said. "But you have to compare that with the privacy you would be forgoing."

Luckey countered: "Civil liberties are extremely important. But we have to realize that we don't have the luxury of what we had in our lives prior to 9/11."

Marvin Badler, an aviation security consultant, added, "We have to find the bad guy before he gets to the airport."

The TSA says it employs a multi-level approach to inspect passengers and the items they carry. It has placed explosive-detection machines at major airports. It also has installed high-tech "puffer" machines that detect explosive particles on a person.

The agency also has a surveillance program in which law enforcement officials in plain clothes monitor the behavior of passengers at airports.

The TSA had hoped to launch a nationwide Registered Traveler program, allowing passengers to funnel quickly through security after paying a fee to receive a background check. But the program, which has been operating on an experimental basis in Orlando, has been put on hold because the TSA wants more input from airports.

Meanwhile, the TSA continues to explore new technologies, said White, the agency spokesman.

"We're always interested in employing new technologies at the checkpoint," he said.

Jim Cannady, an associate professor at the Nova Southeastern University graduate school of computer and information science, said the more levels of security, the better.