Here is a Google-Translated article from a German blog saying Chinese agents might have collected F-117 wreckage in 1999 in Serbia when it was shot down.

Chinese stealth technology came from the Americans

n 11 January completed the first Chinese fighter jet with stealth technology its maiden flight and was presented to the world with it. The aircraft named Chengdu J-20 shows the great progress which the Chinese in modern weapons technology have made.

The question which many military experts will now that came from the know-how to protect yourself from the enemy's radar "invisible" to do? The answer could be, indirectly by the Americans themselves, because during the Balkan war of 1999, the Serbs shot down an American F-117 Nighthawk and the Chinese came by the debris ran to analyze and "reverse engineering" rebuild.

Read all: [link to translate.google.com]