In an airplane hangar north of Fort Worth, technicians are preparing to mount a fire-hydrant-shaped device onto the belly of an American Airlines Boeing 767. It is an effort that could soon turn into a more than $10 billion project to install a high-tech missile defense system on the nation's commercial planes.
The Boeing 767 -- the same type of plane that terrorists flew into the World Trade Center -- is one of three planes that, by the end of this year, will be used to test the infrared laser-based systems designed to find and disable shoulder-fired missiles. The missiles have long been popular among terrorists and rebel groups in war zones around the world; the concern now is that they could become a domestic threat.
The tests are being financed by the Department of Homeland Security, which has been directed by Congress to move rapidly to take technology designed for military aircraft and adapt it so it can protect the nation's 6,800 commercial jets. It has so far invested $120 million in the testing effort, which is expected to last through next year.
Yet even before the tests begin, some members of Congress, and several prominent aviation and terrorism experts, are questioning whether the rush to deploy this expensive new antiterrorism system makes sense.
Homeland Security officials have repeatedly cautioned that no credible evidence exists of a planned missile attack in the United States. But there is near unanimity among national security experts and lawmakers that because of the relatively low price and small size of the missiles, as well as the large number available on the black market, they represent a legitimate domestic threat.
The concern is not just for the lives that would be lost in the shoot-down of a single plane, proponents say. It is for the enormous economic consequences that would result if the public were to lose confidence in flying.
''We are long overdue for a passenger aircraft to be taken down by a shoulder-launched missile,'' said Representative John L. Mica, Republican of Florida, who is pushing for the systems to be installed. ''We have been extremely, extremely lucky.''
But a significant contingent of domestic security experts say the administration's focus on these missiles may be misdirected. They cite the broad range of ways that terrorists might strike next and point to studies showing that shoulder-fired missiles -- the most popular of which are American-made Stingers and Soviet-made SA-7's -- present less of a threat at airports than do truck bombs or luggage bombs.
''People have probably assumed that these kinds of weapons would work with much greater certainty,'' said K. Jack Riley, the director of the public safety and justice program at the Rand Corporation, a nonprofit research organization that has studied threats from shoulder-mounted missiles. ''This is not as big a threat as people might think.''
Northrop Grumman and BAE Systems are competing to build the devices, which rely on plane-mounted sensors that detect heat-seeking missiles and then automatically fire infrared lasers to jam or confuse the missiles' guidance systems. The defense would be used for about a 50-mile area around airports, while planes land or take off.
The American Airlines Boeing 767 and two jets owned by Northwest Airlines and FedEx will be tested to determine whether they remain as airworthy with the new technology aboard and to figure out if, in simulated attacks, the defense system is reliable. For now, no passengers will be aboard.
Shoulder-fired missiles were introduced by the Americans and the Soviets in the 1960's to protect ground forces. A recent Congressional study found that more than 350,000 existed in government arsenals worldwide. But they also are a favorite of rebel groups and terrorists. At about six feet long and 50 pounds, they are easy to transport, and older models can cost only a few hundred dollars.
Adapted from military technology, Guardian is designed to detect a missile launch and then direct a laser to the seeker system on the head of the missile and disrupt its guidance signals.