BBC reporter detained investigating China hacking
19 February 2013 Last updated at 12:55 GMT
A secretive branch of China's military is most likely one of the world's "most prolific cyber espionage groups", a US cyber security firm has said.
Mandiant said it had traced the hacking activities of APT1 to the site of a 12-storey building in the Pudong area of Shanghai.
The BBC's John Sudworth went along to investigate but was stopped and briefly detained.