Overshadowed by the debate over passenger screening is a terrorist threat as real and elusive as a hijacking  — the possibility of terrorists using shoulder-fired missiles (search) to down a commercial jet, either on the tarmac or in the air.

“We could face a grim future if we do nothing,” security expert David Forbes of BoydForbes Security (search) told an audience this week at the Cato Institute (search), which sponsored a discussion on shoulder-fired missiles, or man-portable air defense systems (MANPADs)(search).

Forbes pointed to a thwarted missile attack against an Israeli charter plane off the coast of Kenya in November 2002 — the first documented action against a commercial airliner outside of a war zone. He called the incident a “precursor” for anti-U.S. attacks in the future.

“There are hundreds of thousands of [MANPADs] proliferated throughout the world, that’s the threat we need to deal with,” said Alvin Schnurr, security expert with defense contractor Northrop Grumman (search).

Testifying before the Senate Select Committee on Intelligence on Tuesday, Vice Admiral Lowell E. Jacoby, director of the Defense Intelligence Agency (search), emphasized the threat.

“A MANPAD attack against civilian aircraft would produce a large number of casualties, international publicity and a significant economic impact on civil aviation,” Jacoby said. “These systems are highly portable, easy to conceal, inexpensive, available in the global weapons market, and instruction manuals are on the Internet.”

Experts say that since the late 1950s, hundreds of thousands of MANPADs have been manufactured worldwide, with countless numbers frequently made and distributed to nefarious sources by cash-strapped countries.

The United States itself provided the Afghan resistance thousands of its Stinger-style (search) missiles to fight the Soviets in the 1980s. The fighters used the shoulder-fired rockets to damage or destroy hundreds of Soviet aircraft.

Now, those weapons are bought and sold on the black market and used against U.S interests, experts contend. Forbes said about 27 terror groups have access to the weapons, which are still being manufactured in 20 countries.

MANPADs are typically about five feet long and weight approximately 30 to 40 pounds — they can fit in a golf bag. They can be launched from anywhere within an airport perimeter, and even outside the property — as far as 10,000 feet from their target.

James Carafano, a counter-terrorism expert at the Heritage Foundation (search), said MANPADS are easy to find and smuggle, which makes them a very real threat to airplanes.

“Is it going to happen? Yeah, it’s just a matter of time,” he told Foxnews.com.

The weapons — most of which use guided infrared sensors (search) to find their target — go for about $5,000 apiece on the black market. U.S troops are offering hundreds of dollars to buy back missiles from Iraqis today, Forbes said.

While efforts are being made in the private sector and the Department of Homeland Security to perfect anti-missile systems to protect U.S aircraft, some proponents are demanding more energy and resources, and insist on having it sooner than later.

“They have not moved as aggressively as they need to,” said Rep. Jim Langevin, D-R.I., a member of the House Homeland Security Committee. Langevin told Foxnews.com that a program currently testing onboard, anti-missile systems to protect planes from MANPADs needs more money than the $60 million budgeted for it in 2004.

Langevin has co-sponsored legislation that would require such onboard systems, much like the countermeasures used by military planes in Iraq on carrier jets. He admits that it is an expensive endeavor that does not have the attention of Congress.

“I’m hoping it would be taken more seriously,” he said.

But Chuck Pena, director of defense policy studies at Cato, said equipping the more than 6,600 domestic commercial airplanes with complex anti-missile systems could cost upwards of $20 billion. And even then, no system is fail-safe.

It is necessary to do something, however, including cutting wasteful or obsolete defense programs to pay for it, he said.

“We’d be closing a terrorist loophole,” Pena said, adding that since tougher passenger screening and airport security have been put into place since the Sept. 11, 2001, terror attacks, terrorists are inevitably looking at other ways to exploit vulnerabilities.

“Commercial aircraft are not equipped with countermeasures and commercial pilots are not trained in evasive measures. An attack could occur with little or no warning, ” Jacoby testified. “Terrorists may attempt to capitalize on these vulnerabilities. ”

MANPADs have been used in at least four attacks, including two hits against U.S targets in Iraq, one of which downed a military transport plane in January. No deaths resulted.

Last August, the FBI — with assistance from Russian and British authorities — thwarted an attempt by a man with links to Al Qaeda (search) to purchase a MANPAD in Russia and smuggle it into the United States.

Schnurr said testing is in the works on a ground-based laser system that would seek and destroy any projectiles launched at an aircraft at or above an airport. But like the onboard systems, as well as all of the other considerations dealing with missile prevention, the costs are somewhat inestimable.

But Carafano said countermeasures need to be launched now so that they will be effective in the short-term.

“Let’s assume that a plane is going to be taken down, because that is going to happen,” said Carafano. “So let’s sit down and figure out what we’re going to do now, not after a plane gets shot down.”