The threat from portable missiles is one such threat.

But portable missiles remain a formidable threat to commercial aviation.

The arrest followed a series of urgent warnings from Bush administration officials that terrorists might use portable missiles against commercial aircraft.

The use of portable antiaircraft missiles to protect a large, immovable target like a stadium is not unreasonable, several military experts said.

They're firing portable missiles at us from the ship!

The worldwide inventory of portable surface-to-air missiles is estimated at 700,000, according to government officials and private weapons specialists.

"There's always the possibility of these manned portable surface-to-air missiles," the general added.

The cause of the crash remained unclear, with some officials saying the helicopter appeared to have been shot down by a portable surface-to-air missile.

But portable missiles were fired at incoming planes several times in recent weeks, one senior official said.

Moderating security fears is the fact that the portable missiles cannot be fired effectively without training.