Beijing, Reuters—Chinese police have detained the country’s most high-profile Uighur academic, an outspoken critic of official policies in the restive far-western region of Xinjiang, on suspicion of “breaking the law,” the government said on Thursday.

Wednesday’s detention is the latest indication of the government’s increasing hardline stance on dissent surrounding Xinjiang, where a series of violent riots in the past year have killed at least 91 people, rights activists say.

Xinjiang is home to the Muslim Uighur people, who speak a Turkic language. Many resent what they see as oppressive treatment by the government, though Beijing says they are granted wide religious, cultural and linguistic freedoms.

Police in Beijing seized Ilham Tohti, a prominent Uighur economist who has championed the rights of the Uighur community in Xinjiang, at his home. His whereabouts are unknown, his wife and close friend told Reuters.

Tohti has challenged the government’s version of several incidents involving Uighurs, including what Beijing says is its first major suicide attack involving two men from Xinjiang in Beijing’s Tiananmen Square, by pointing out inconsistencies in the official accounts, said Nicholas Bequelin, a senior researcher at Human Rights Watch.

“Ilham is suspected of breaking the law,” Chinese Foreign Ministry spokesman Hong Lei said at a daily news briefing.

“The public security organs have detained him in accordance with the law. The relevant departments will now deal with him in accordance with the law.”

Tohti’s wife, Guzailai Nu’er, said around 30 police from Xinjiang and Beijing arrived at her apartment to seize Tohti and confiscate his computer and books. They did not give her a reason, she said.

“I trust my husband. He didn’t do anything unlawful,” she said in a phone interview from her apartment, which was surrounded by police. “All he has done is write detailed articles researching the population in Xinjiang. There’s nothing else to it.”

Tsering Woeser, a prominent Tibetan writer and friend, said Tohti told her a week ago he had heard that the authorities in Xinjiang had informed Beijing they wanted to arrest him. She said he later heard that Beijing did not approve the arrest.

“At that time, he said that he felt he was in a very dangerous position,” Woeser said.

Tohti, an economics professor at Beijing’s Minzu University which specializes in ethnic minority studies, told Reuters in November that state security agents had physically threatened him for speaking to foreign reporters.

Bequelin said Tohti’s detention reflected a harder stance that Chinese President Xi Jinping has taken on Xinjiang. State media called this a “strategy shift” from development to maintaining social stability.

Bequelin said Tohti could be charged with “incitement for separatism,” which carries a maximum sentence of life imprisonment.

“The test of what constitutes incitement of separatism is anything that opposes ethnic harmony,” he said.

Tohti, Bequelin said, was “an intellectual trying to get the state to see objectively what the situation is.”

“He wants to tell truth to power,” he said. “I cannot see anything that he has written or done that can be construed as endangering state security. But the outcome is a foregone conclusion.”