It's a threat that cannot be ignored. Worldwide, dozens of countries are suspected of having the surface-to-air missiles, and at least 17 terrorist organizations may possess them. Two dozen nonmilitary aircraft have been shot down with missiles in other nations in the past 25 years, and even one attempt on U.S. soil, successful or not, is likely to ruin an already decimated airline industry.

"The fact the government is so up in arms about it indicates it is a legitimate threat," said Matt Schroeder, a research associate with the arms-sales monitoring project of the Federation for American Scientists in Washington, D.C. "It's a sufficient enough threat that policymakers need to come up with something."

The federal Transportation Security Administration has mapped and photographed the perimeters of the busiest American airports to look for potential launching areas for the relatively cheap and easily hidden missiles after rockets missed an Israeli airliner in Kenya last year.

Neither federal nor DIA officials will say if they have increased patrols at Denver's largely rural airfield, citing secrecy as an advantage in combating potential attacks. But Los Angeles airport officials said recent stepped-up National Guard patrols were partly in response to the missile threat, and Homeland Security Secretary Tom Ridge said this month that the government will consider equipping all U.S. airplanes with missile-jamming equipment.

With a range of several miles, the cheap Stinger-type missiles can be fired from unguarded spots anywhere within a vast diameter around major airfields. Equipping the U.S. commercial airline fleet with jammers or decoys could cost up to $10 billion in a time of slashed budgets.

Forcing themselves to think the unthinkable since the 9/11 attacks, security planners find the missile threat an overwhelming puzzle.

"There's something you can do about screening passengers. There's nothing you can do about an external threat like this," said Michel Merluzeau, a security and terrorism consultant for Frost & Sullivan.

"It would be financially catastrophic to the airlines," said Ross Bulla, president of a corporate and government security firm, the Treadstone Group.

With shoulder-fired missiles, military efficiency has become an enemy of passenger safety. The standard in small missiles is the U.S.- built Stinger system, which allows one or two operators to set up and fire within seconds. Stingers can hit an airplane up to 2 miles above ground from 3 miles or more on either side of the jet's flight path.

The heat-seeking missile is considered very accurate yet can be hidden almost anywhere. The 5- foot-long launcher weighs only 13 pounds, the missile another 22 pounds, and all can fit in a van or a car trunk. They are fire-and-forget, meaning a terrorist could aim, fire and leave the area before the missile even hits the aircraft.

Stingers and their Soviet or Chinese copies litter the world like straw blown from a hay truck. The U.S. shipped them to Afghan rebels to knock down Soviet aircraft, and hundreds are still unaccounted for there. The Soviets and Chinese also gave or sold them to clients since they are the perfect tool for rebels fighting an air power.

Security experts say unreliable knockoffs can be had on black markets for $5,000, while high-quality Stingers can be found for $100,000 or less. Smuggling them into the United States would not be a challenge, these experts say, with overwhelmed borders and millions of unsearched containers arriving at American ports.

"Somebody could be in their backyard in Los Angeles and fire a missile. There's no way of knowing it will happen until the missile is fired," Merluzeau said.

The density of Los Angeles is mentioned frequently in Stinger discussions. Denver officials even compare themselves favorably to other airports when making limited comments on the missile threat.

"When you can stand on public property and throw a tennis ball at an airplane, that's a more difficult situation. We have more wide-open spaces that make it more difficult not to be seen from a distance," said Amy Bourgeron, deputy manager of aviation at DIA. Bourgeron declined to give specifics but said perimeter patrol at DIA has increased for a number of threats, not just missiles, since 9/11. Denver also exceeds federal standards in other areas, she said, including body scans to identify airport employees at all worker entrances.

United Airlines, the dominant passenger carrier at DIA, declined to comment, referring questions to federal and industry sources.

Among other measures, the Department of Homeland Security is talking to civilian pilots and aviation-related groups to teach them to identify the missile launchers, to be another set of eyes on airport surroundings, said Brian Roehrkasse, spokesman for the agency.

Roehrkasse acknowledged increased government worries about the risk but added that the threat is general at the moment. "There is no credible intelligence" that terrorists have such weapons in the U.S. or plan to use them for commercial jet attacks, he said.

The agency has compiled statistics, however, to answer public questions about the threat. Since 1978, there have been 35 attempts worldwide to shoot down civilian aircraft with such missiles; 24 of those were successful, killing more than 500 people. But only six of the attempts were on multi-engine jets, and five of those escaped with little or no damage. Nearly all strikes have been on propeller-driven aircraft, Roehrkasse said.

The most effective prevention the government can do right now, Merluzeau said, is the gumshoe investigative work that has already paid off in preventing other terrorist acts inside U.S. borders.

"The first line of defense is really the intelligence agencies keeping an eye on potential operators" of the missiles, he said. The government has received a lot of data on missile threats from captured Taliban and al-Qaeda fighters at the U.S. base and prison at Guantanamo Bay, Cuba, Merluzeau said.

Equipping the planes to defend themselves is a hugely expensive proposition but one that Congress and federal officials will ponder deeply in the next year. Military planes release metal chaff or use other countermeasures to foil the heat-seeking cameras at the missile tips. Some reports said the Israeli plane used such measures.

But outfitting a commercial jet may cost $1 million, requiring $6 billion to $10 billion to protect the entire U.S. commercial fleet. One alternative is to use the defense technology on a selection of planes to complicate the task of terrorists, said Steve Hansen, spokesman for a congressional transportation committee that recently held closed-door hearings on missile threats. That would be similar to the U.S. air marshal program, in which armed agents accompany some flights, in part to confuse terrorists about which planes are protected and which are not.

Where those billions are best spent will be the key question as lawmakers debate the missile-jamming devices, said William Lahneman, a former naval commander and a program coordinator for the Center for International Security Studies at the University of Maryland. Searching more ship containers in ports could "do a lot of good" in keeping weapons of mass destruction out of the country, Lahneman said. Since Congress is already straining to find enough money to fight terrorism, the benefits of port searches must be weighed against expensive air defenses aimed at saving far fewer lives in a single attack.

Without a solution as obvious as, for example, bomb detectors at baggage counters, the government will have to focus on three areas, Roehrkasse said. Diplomats and agents will try to limit the supply of missiles sold or provided to terrorists overseas; researchers will consider the cost and effectiveness of defending each aircraft; and U.S. security officials will continue following terrorist suspects and asking local residents or workers to look out for weapons.

Merluzeau and other security consultants tell worried clients that while a missile attack is highly possible in the next couple of years, their chances of being on that one plane at that one airport at the wrong time are "very low." Clients who still worry might consider using smaller, lesser-known airports for their flights, Bulla said.

Once thought of, though, the missiles are hard for travelers to dismiss entirely, Merluzeau said.

"We don't like the idea of thinking about this for the first and last five minutes of a flight," he said.