Most airline passengers are not checked for explosives before boarding planes. The current generation of machines that screen passengers for weapons can't detect plastic explosives.

While airlines have a congressionally mandated deadline of Jan. 18 for having a system in place to inspect all checked baggage for explosives, walk-through devices that could detect them on passengers are still in the development stage.

"It's a hole that needs to be looked at," said Capt. John Cox, executive air safety chairman of the Air Line Pilots Association.

Airline experts say the only way to prevent a passenger from bringing an explosive on board is singling out potential terrorists through computerized profiles and then calling in bomb-sniffing dogs or conducting body and clothing searches.

"Profiling is the key," said David Stempler, president of the Air Travelers Association, an advocacy group. "Security is composed of two parts. The first is who are you and the second is what are you carrying."

Authorities have worried that terrorists might place bomb-laden bags on airplanes -- like the suitcase that blew up Pan Am Flight 103 -- without boarding the plane themselves. Today, luggage is not loaded on international flights unless the passenger also boards.

That passengers might smuggle explosives aboard on their bodies or in clothing didn't get much attention until a man boarded a flight in Paris on Saturday and was later arrested with what the FBI said was an explosive hidden in one of his shoes.

"In terms of a suicide bombing on an airplane, that is a scenario that has not really been planned for in a serious way," said Paul Hudson, executive director of the Ralph Nader-affiliated Aviation Consumer Action Project. Hudson's daughter was killed on Flight 103 in 1988.

"With what we know about the use of suicide bombers, we have to assume that is a threat, not just a remote possibility," Hudson said.

The Federal Aviation Administration ordered airlines and airports on Sunday to take steps to prevent passengers boarding planes with explosives hidden in shoes. The agency warned airlines earlier this month that terrorist hijackers might hide weapons in their shoes.

"All of us have to be on alert," the top Republican on the Senate Intelligence Committee, Sen. Richard Shelby of Alabama, said Sunday on CBS' "Face the Nation." "We've known there would probably be reprisals by terrorists or terrorist groups because of what's happened in Afghanistan, our success there. And I think this is just one incident."

One security procedure put into effect after the Sept. 11 attacks did work -- relying on passengers and flight attendants to subdue a suspect.

"The fact that the passengers jumped in shows it has to be a group effort," said Lori Bassani, a spokeswoman for the Association of Professional Flight Attendants. "It's everybody's problem and everybody's job to be vigilant and to help out."

Hudson said planes need to be strengthened so that a small amount of explosives cannot bring down an aircraft, including installing bomb-resistant cargo containers and overhead bins.

"The plane needs to be protected so that, even if explosives get on board, the small amount you can carry on your person would not destroy the aircraft," he said.