It has been known for years that thousands of light and lethal shoulder-fired missiles are in black-market circulation. What is not known is exactly who has them and whether many have fallen into the hands of terrorists or criminals. A worrisome puzzle, it explains why the United States and Russia signed an agreement Thursday to cooperate in destroying surplus Soviet-era SA-7s and other portable anti-aircraft missiles. The smallest of these are durable, relatively cheap and easy to smuggle. The United States also has understandings with several other countries, including Nicaragua, Bosnia, Cambodia and Liberia, for Washington to provide technical assistance or money to destroy anti-aircraft missiles. The State Department estimates that about 1 million shoulder-fired anti-aircraft missiles have been produced worldwide since the 1950s. The number believed to be in the hands of "nonstate actors,'' such as terrorist groups, is ``in the thousands,'' the department says. "What's driving this is concern about the threat to commercial aviation,'' said Wade Boese, research director at the private Arms Control Association. A single successful missile attack on a passenger plane could paralyze the airline industry, at enormous economic loss, he said. There has been only one known attempt against a commercial airliner outside of a war zone. [more]