Sun Xiaodi, a former employee of the Gansu No 792 Uranium Mine in northwestern China, met journalists on 28 April while he was in Beijing to petition authorities about what he said was serious pollution from the mine.
He said residents near and downstream of the plant suffered a high incidence of cancerous tumors, leukemia, birth defects, miscarriages and other afflictions.
A day after the meeting, Sun was bundled into an unmarked car near Beijing's southern railway station and has not been heard from since, Human Rights in China (HRIC) said, citing numerous witnesses.
It said that later on the same day, several plainclothes police officers searched the Beijing home of a friend of Sun, whom they then took to a State Security Bureau office.
They reportedly told the friend that Sun was a "wanted criminal" and that he had committed a "very serious crime related to state secrets."
Police produced Sun's cell phone, wallet, telephone diary and other personal belongings, the rights group said.
Sun's daughter, Sun Haiyan, has inquired about her father's whereabouts many times, but has been repeatedly told by Beijing authorities that they know nothing, HRIC said.
"HRIC condemns the unlawful abduction and secret detention of Sun Xiaodi, which violate both his Chinese constitutional rights and his human rights"
Human Rights in China
The Beijing Public Security Bureau refused to comment on Friday, and the State Security Bureau could not be reached.
The No 792 Uranium Mine in Gansu province's Diebu county was established under the State Nuclear Industry Department as one of China's most important sources of uranium.
But it was "closed as a matter of policy" in 2002 on the basis of mine-exhaustion.
After the closure, mine employees accused mining and Nuclear Industry Department officials of plundering employee and state assets and damaging the environment, but their complaints were ignored by the authorities, HRIC said.
Local medical workers report that nearly half of all deaths in the area are from some form of cancer, but patients case histories are routinely altered because of "state secrets" concerns, HRIC said.
Sun began reporting the health concerns to the Nuclear Industry Department in 1988. But instead of an official response, he was fired and his family was put under surveillance, the group said.
"HRIC condemns the unlawful abduction and secret detention of Sun Xiaodi, which violate both his Chinese constitutional rights and his human rights," the group said in a statement.
"HRIC calls on the Chinese authorities to immediately release Sun Xiaodi from custody, and as a matter of urgency to address the dangerous environmental contamination and severe health hazards to humans and animals near the No. 792 Uranium Mine."