(CBS) The FBI told U.S. police departments an empty launcher for a shoulder-fired anti-aircraft missile found near a military base in Saudi Arabia was linked to al Qaeda, but cautioned there were no signs terrorists planned to fire on U.S. commercial jetliners.

Instead, the FBI indicated it believes al Qaeda may try to use such weapons against U.S.-led military forces in the region.

"We take very seriously the fact that our opponents do have...shoulder-fired surface to air missiles," U.S. Marine Corps General Peter Pace, vice-chairman of the Joint Chiefs of Staff told a Pentagon briefing, "and we take precautions on the ground and in the air any time we have our aircraft arriving or departing."

The FBI message, marked "law enforcement sensitive," noted that U.S. planes have used the Prince Sultan Air Base, south of the Saudi capital Riyadh, since the 1991 Gulf War. The FBI said Thursday it was unknown how long the missile tube had been there before its discovery by Saudi soldiers about three weeks ago.

"There are no reports by any U.S. aircraft or any aircraft that we can identify of any sightings of surface to air missile firings," Pace said. "That does not mean it was not fired. It simply means we do not know if that particular weapon was fired at that location or simply dropped off there."

The FBI sent the message May 22 urging state and local police departments to remain vigilant, but cautioned there was no hint of any attack plot. "We have no information to indicate al Qaeda is planning to use any type of missile or weapons systems against commercial aircraft in the U.S.," the bulletin said.

Saudi security guards at the base found the 4-foot-long launcher for a Soviet-made SA-7 missile about two miles from the nearest runway, inside the base's outer perimeter fence and near an inner fence.

A cover on the front of the launch tube was intact, but there were scorch marks on the back of the tube, indicating it could have been used to fire or try to fire a missile. The discovery puzzled and worried U.S. military officials. No pilots reported seeing, hearing or detecting any missiles, and officials said there were no reports of any threats against the air base.

About 4,500 U.S. troops and an unspecified number of American warplanes use the desert base.