Attorney Wang Quanzhang, who defended political activists and victims of land seizures, has had no contact with the outside world since he disappeared in a 2015 police sweep aimed at courtroom critics of Communist authorities.
Wang has since been charged with "subversion of state power" but authorities have blocked lawyers from visiting him.
"Maybe something horrible happened to him in jail and that is why authorities don't want anyone to find out," his wife Li Wenzu told AFP.
"It's been 1,000 days. I don't know if he is alive or dead."
Li and a small group of supporters set off Wednesday on a march from Beijing to the "No. 2 Detention Centre" in the northeastern city of Tianjin, where officials last said he was being held.
They pressed on even during a freak snowstorm and plan to reach Tianjin by next Friday.
"We want to see the president of the court. We want to see the presiding judge. It would be good if someone came out to tell us what Wang Quanzhang's case was all about. If he committed no sin, they should release him," she said.
Li wore a black sweatshirt with the slogan "Free Quanzhang" and a cap embroidered with the words of the last letter Wang sent his parents.
Fearing reprisal, he told them that becoming a human rights lawyer was not a "reckless decision" but a response to a call from within.
For nearly three years, Li has made dozens of freedom of information requests to police -- which have been sent back unanswered -- and she has visited the complaints office of the Supreme People's Court in Beijing weekly to no avail.
Instead of answers, she has been put under constant police surveillance.
Tears streamed down her cheeks as Li talked about how their five-year-old son is afraid of the state security officers who have moved into an apartment below their home.
China's 'lies'
For several days beginning on July 9, 2015, more than 200 Chinese human rights lawyers and activists were detained or questioned in the largest clampdown on the legal profession in recent history.
While the majority were released on bail, a handful -- including prominent lawyers Xie Yang and Li Heping -- were convicted of various crimes and sentenced to up to seven years in prison.
Wang's case is unusual because no trial date has even been announced. He is the last person in the so-called "709 crackdown" to remain in legal limbo.
When AFP called the Tianjin No. 2 Detention Centre on Thursday, a man who picked up said: "You should not ask about this matter."
Frances Eve, a researcher for the charity Chinese Human Rights Defenders, said the tireless campaigning of family members of detained lawyers had drawn international attention to China's "lies" about its legal system.
"Li shouldn't be forced to wait years and walk miles to receive basic information about her husband's status. Wang Quanzhang should be released and reunited with his family," Eve said.
China's ruling Communist party has repeatedly pledged to implement the "rule of law", but analysts say the crackdown shows the limits of that promise.
The country's courts are tightly controlled by the party, with forced confessions often used as evidence and guilty verdicts delivered in more than 99.9 percent of criminal cases.
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