BEIJING (Reuters) - A well-known Chinese legal activist has been missing for several days, according to his wife, who said he had last made contact on a trip to a central province after being tracked by state security agents.

Jiang Tianyong, 45, has spoken out about a government crackdown on legal defenders and has been involved in high-profile cases of dissidents who have irked Chinese authorities, including the case of blind activist Chen Guangcheng, who got out of China after he fled to the U.S. embassy in 2012.

Jiang’s wife, Jin Bianling, said she and friends had been unable to contact him since Monday night, after he traveled to the city of Changsha, in central Hunan province, to visit relatives of an arrested human rights lawyer, Xie Yang.

After confirming to a friend that he had bought a return train ticket to Beijing, Jiang went missing. State security agents had been tracking Jiang, Jin said.

“State security had gone to his brother’s house in the past to ask about him,” Jin told Reuters by telephone from the United States, where she has lived since 2013, due to what she called pressure on the family because of her husband’s work.

“So we’re very worried that he was taken away from the train station or that there was some kind of accident,” Jin said.

Police in Zhengzhou, in the central province of Henan, where Jiang has family who tried to make a report of a missing person, told Reuters by telephone they were unaware of the situation, as did China’s Foreign Ministry.

An official at the media office of the Ministry of Public Security in Beijing declined comment, saying it could not comment on individual cases.

Authorities have refused to renew Jiang’s law license and he has been detained several times in the past.

Dozens of people linked to a Beijing law firm have been detained or prosecuted in a crackdown on dissent since July 2015, as President Xi Jinping’s administration has tightened control, citing a need to boost national security and stability.

Many of those lawyers and activists have been sentenced for subverting the government, drawing the condemnation of international rights groups and governments.

China consistently rejects criticism of its human rights record and says it is a country ruled by law.

Authorities have accused dissident lawyers and legal activists of orchestrating protests outside courts, politicizing ordinary legal cases, and conspiring with “foreign forces” to undermine China’s ruling Communist Party.