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

They also said the attacks, aimed at stealing trade secrets and computer codes and capturing e-mail 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.

Computer security experts, including investigators from the National Security Agency, have been working since then to pinpoint the source of the attacks. Until recently, the trail had led only to servers in Taiwan.

If supported by further investigation, the findings raise as many questions as they answer, including the possibility that some of the attacks came from China but not necessarily from the Chinese government, or even from Chinese sources.