Nightmare scenario.

(ABC News) — U.S. officials had once thought there was little chance that terrorists could get their hands on many of the portable surface-to-air missiles that can bring down a commercial jet liner.

But now that calculation is out the window, with officials at a recent secret White House meeting reporting that thousands of them have gone missing in Libya.

“Matching up a terrorist with a shoulder-fired missile, that’s our worst nightmare,” said Sen. Barbara Boxer, D.-California, a member of the Senate’s Commerce, Energy and Transportation Committee.

The nightmare has been made real with the discovery in Libya that an estimated 20,000 portable, heat-seeking missiles have gone missing from unguarded Army weapons warehouses.

The missiles, four to six-feet long and Russian-made, can weigh just 55 pounds with launcher. They lock on to the heat generated by the engines of aircraft, can be fired from a vehicle or from a combatant’s shoulder, and are accurate and deadly at a range of more than two miles.