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."
Efforts to get comment from China's defense ministry and the Pentagon were unsuccessful.
China's multi-role stealth fighter - known as the Chengdu J-20 - made its inaugural flight Jan. 11, revealing dramatic progress in the country's efforts to develop cutting-edge military technologies.
Although the twin-engine J-20 is at least eight or nine years from entering air force inventory, it could become a rival to America's top-of-the-line F-22 Raptor, the successor to the Nighthawk and the only stealth fighter currently in service.
Despite Chinese President Hu Jintao's high-profile visit to the United States this week, many in Washington see China as an economic threat to the U.S. and worry as well about Beijing's military might.
Parts of the downed F-117 wreckage - such as the left wing with US Air Force insignia, the cockpit canopy, ejection seat, pilot's helmet and radio - are exhibited at Belgrade's aviation museum.
"I don't know what happened to the rest of the plane," said Zoran Milicevic, deputy director of the museum. "A lot of delegations visited us in the past, including the Chinese, Russians and Americans ... but no one showed any interest in taking any part of the jet."
Zoran Kusovac, a Rome-based military consultant, said the regime of former Serbian President Slobodan Milosevic routinely shared captured Western equipment with its Chinese and Russian allies.
"The destroyed F-117 topped that wish-list for both the Russians and Chinese," Kusovac said.
Russia's Sukhoi T-50 prototype stealth fighter made its maiden flight last year and is due to enter service in about four years. It is likely that the Russians also gleaned knowledge of stealth technology from the downed Nighthawk.
The F-117, developed in great secrecy in the 1970s, began service in 1983.
While not completely invisible to radar, its shape and radar-absorbent coating made detection extremely difficult. The radar cross-section was further reduced because the wings' leading and trailing edges were composed of nonmetallic honeycomb structures that do not reflect radar rays.
Kusovac said insight into this critical technology, and particularly the plane's secret radiation-absorbent exterior coating, would have significantly enhanced China's stealth know-how.
Alexander Huang of Taipei's Tamkang University said the J-20 represented a major step forward for China. He described Domazet-Loso's claim as "a logical assessment."
"There is no other stronger source for the origin of the J-20's stealthy technology," said Huang, an expert on China's air force. "The argument the Croatian chief-of-staff makes is legitimate and cannot be ruled out."
The Chinese are well-known perpetrators of industrial espionage in Western Europe and the United States, where the administration has also been increasingly aggressive in prosecuting cases of Chinese espionage.
Western diplomats have said China maintained an intelligence post in its Belgrade embassy during the Kosovo war. The building was mistakenly struck by U.S. bombers that May, killing three people inside.
"What that means is that the Serbs and Chinese would have been sharing their intelligence," said Alexander Neill, head of the Asia security program at the Royal United Services Institute, a defense think tank in London. "It's very likely that they shared the technology they recovered from the F-117, and it's very plausible that elements of the F-117 got to China."