Shoulder-fired missiles are a threat to civilian jetliners, but putting antimissile defenses on planes is far too expensive to be practical now, according to a study by the RAND Corporation.

Since January 2004, the Department of Homeland Security has invested $6 million to have plans drawn up for on-board antimissile systems for civilian jetliners.

Scheduled passenger flights are unlikely to be affected by the Pentagon's commandeering of 38 civilian jetliners.

On July 3, a United States warship mistook an Iranian civilian jetliner for an Iranian fighter plane and blew it out of the sky with a heat-seeking missile.

The need for an improved system was demonstrated by the downing in July of an Iranian civilian jetliner by the Navy cruiser Vincennes in the Persian Gulf.

The Malaysian Defence Ministry, reiterated by various netizens, also pointed out the fact that chartering civilian jetliners are also a common practice in other countries, including those of NATO.

Had the accord been in effect in 1983, it might have prevented the shooting down of a South Korean civilian jetliner by a Soviet fighter.

The evidence seized by the West German authorities showed that group members were well on the way to bombing a civilian jetliner.

Military officials said Captain Rogers was uncertain about the identity of the target and was concerned that it might be a civilian jetliner.

Administration officials said the previously undisclosed incident helps explain why the captain of the Vincennes thought he might be facing an attack from an F-14 on Sunday, when he shot down a civilian jetliner.