An elite military unit of Chinese hackers is likely behind a wave of attacks on U.S. government and business computer systems, according to a report released on Tuesday by the American security firm Mandiant.

Analysts traced a series of attacks to a 12-story building in the Pudong district of Shanghai. They concluded that the building, which likely holds hundreds or even thousands of employees, is almost certainly the headquarters of China’s secretive cyber war division, the People’s Liberation Army Unit 61398.

A state-owned telecommunications company provided special fiber optic cable to the building in the name of national defense, according to the 74-page report. The analysts said that the hackers, who are required to be proficient in English, likely have a sizable support staff of linguists, researchers and industry experts.