The information involved routine instructions on how officials were to safeguard social stability during the 15th anniversary of the June 4, 1989, democracy movement.

Shi's case alarmed critics of the Chinese government because his posting did not reveal the sender or the source of the information. That meant the authorities had no more to go on when they began their investigation than an anonymous posting on a U.S.-based Web site.

Using investigative techniques that were not revealed during Shi's trial, Beijing state security officials pinpointed the Chinese source of the e-mail to Democracy Forum that contained the information, according to the people involved in Shi's defense. The people asked to remain anonymous because evidence introduced in Shi's trial was declared secret.

How investigators obtained that information - either by gaining access to the computers at Democracy Forum in New York, or by retroactively searching the content of e-mails that passed through China's Internet servers - was presented to judges in a secret addendum to the prosecution's evidence. Shi's defense lawyers did not have access to that evidence, the people said.

Yahoo's role remains murky, in part because the company has declined to provide any information about its cooperation with Beijing authorities.

It is unclear whether the company responded voluntarily to a request from Beijing state security, or provided data only when confronted with a court order.

Whether the information that Yahoo provided proved critical in identifying Shi, or merely supplementary to the case, also remains unclear. This is because it is not known whether the initial intelligence the Beijing authorities gathered identified Shi personally, or merely the Internet access point, or IP address, where the e-mail to Democracy Forum originated.