On the outskirts of Shanghai, in a run-down neighborhood dominated by a 12-story white office tower, sits a People’s Liberation Army base for China’s growing corps of cyberwarriors.

The building off Datong Road, surrounded by restaurants, massage parlors and a wine importer, is the headquarters of P.L.A. Unit 61398. A growing body of digital forensic evidence — confirmed by American intelligence officials who say they have tapped into the activity of the army unit for years — leaves little doubt that an overwhelming percentage of the attacks on American corporations, organizations and government agencies originate in and around the white tower.