Balkan military officials and other experts have told The Associated Press that in all probability the Chinese gleaned some of their technological know-how from an American F-117 Nighthawk that was shot down over Serbia in 1999.

Nighthawks were the world's first stealth fighters, planes that were very hard for radar to detect. But on March 27, 1999, during NATO's aerial bombing of Serbia in the Kosovo war, a Serbian anti-aircraft missile shot one of the Nighthawks down. The pilot ejected and was rescued.

It was the first time one of the much-touted "invisible" fighters had ever been hit. The Pentagon believed a combination of clever tactics and sheer luck had allowed a Soviet-built SA-3 missile to bring down the jet.

The wreckage was strewn over a wide area of flat farmlands, and civilians collected the parts — some the size of small cars — as souvenirs.

"At the time, our intelligence reports told of Chinese agents crisscrossing the region where the F-117 disintegrated, buying up parts of the plane from local farmers," says Adm. Davor Domazet-Loso, Croatia's military chief of staff during the Kosovo war.

"We believe the Chinese used those materials to gain an insight into secret stealth technologies ... and to reverse-engineer them," Domazet-Loso said in a telephone interview.

A senior Serbian military official confirmed that pieces of the wreckage were removed by souvenir collectors, and that some ended up "in the hands of foreign military attaches."