(CNN) -- An American cybersecurity firm has linked one of the world's most prolific groups of computer hackers to the Chinese government, saying in a new report that an extensive cyber-espionage campaign is being waged from a location near Shanghai. The security firm, Mandiant, detailed the allegations in a 60-page report published Tuesday that describes the group's tactics and history over a six-year period. Mandiant said it has observed the group of hackers -- called the "comment crew" -- systematically steal hundreds of terabytes of data from at least 141 organizations across 20 industries around the world since 2006. Mandiant claims the activity can be traced to four networks near Shanghai -- with some operations taking place in a location that is also the headquarters of Unit 61398 -- a secret division of China's military.
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