A series of online attacks on Google and dozens of other US corporations have been traced to computers at two educational institutions in China, including one with close ties to the Chinese military, people involved in the investigation said.

They also said the attacks, aimed at stealing trade secrets and computer codes and capturing the e-mails of Chinese human rights activists, may have begun as early as April, months earlier than previously believed. Google announced on Jan. 12 that it and other companies had been subjected to sophisticated attacks that probably came from China.