News implicating Yahoo in the imprisonment of Jiang Lijun in 2003 surfaced on the eve of a in Washington.
Video: Fighting Yahoo's China policies
Lawyer, family of dissidents speak out.
It was the third such case involving the U.S. Internet giant.
that led to an eight-year prison term for Li Zhi for subversion in 2003 and of , who was accused of leaking state secrets abroad and jailed last year for 10 years.
Thesaid it had obtained a copy of the verdict showing that Yahoo Holdings in Hong Kong helped Chinese police identify Jiang by confirming that the e-mail account ZYMZd2002 had been used jointly by Jiang and another prodemocracy activist, Li Yibing.
"Little by little, we are piecing together the evidence for what we have long suspected, that Yahoo is implicated in the arrest of most of the people that we have been defending," the group said.
"We hope this Internet giant will not, as it has each time it has been challenged previously, hide behind
But the watchdog conceded that the access code could also have been provided by Li, who is suspected of having been a police informer in the case.
Yahoo could not immediately be reached for comment. The company has defended itself in the past, saying it had to abide by local laws.
The 40-year-old Jiang was accused of seeking to use "violent means" to impose democracy, Reporters Without Borders said.
Police believed that Jiang to be the leader of a small group of Internet dissidents, including Liu Di, a university student who was detained for one year and released in November 2003, after police decided against pressing charges.
The case is the latest in a string of examples that highlight the, the world's No. 2 Internet market.
Web search giant Google has come under fire for saying it would, bowing to conditions set by Beijing.
In December,belonging to outspoken blogger Michael Anti under Chinese government orders.
China has intensified a crackdown on the media in the past year, firing newspaper editors, arresting journalists and closing publications.