Twelve-storey building in Pudong, Shanghai
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BBC reporter detained investigating China hacking

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.

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