A Mongolian activist is still missing nearly a week after his release from prison.

China is well known for locking up political troublemakers for long jail terms without regard for legal niceties. Less well known is that political prisoners often fail to regain their freedom even when those terms formally expire.

A case in point is that of Hada, an ethnic Mongolian activist whose whereabouts remain unknown nearly a week after his scheduled release, on Friday, from a 15-year jail sentence. His wife and son have also been missing since they were detained this month ahead of Mr. Hada's release.

Mr. Hada, who like many Mongolians goes by a single name, is the founder of the Southern Mongolian Democracy Alliance, a group that promoted Mongolian culture and civil rights in China's Inner Mongolia Autonomous Region. He also ran a Mongolian studies bookstore in Hohhot, the region's capital, where he met fellow activists. Arrested in 1995 on charges of separatism and espionage (i.e., talking to foreign media), he has since been behind bars.

While Mr. Hada is hardly the first political prisoner to have his sentence extended arbitrarily, his case is especially strange. On Sunday, a security official delivered to Mr. Hada's sister-in-law Naraa a compact disc with "proof of life"—five photos showing Mr. Hada seated with his wife and son before a table of food. In one photo the three are seen offering a toast toward the camera.

Ms. Naraa told us by phone yesterday that local officials told her that Mr. Hada and his family are safe and in police custody. But they wouldn't tell her where they are being held or when they might be allowed to leave. "Our primary concern is for Hada's health and safety," she said.

On why Mr. Hada is being held beyond his official sentence, authorities suggested to Ms. Naraa that it has taken longer than expected to negotiate the conditions of his freedom. Earlier this year, Mr. Hada suggested to his family that he might try to sue the Chinese government for imprisoning him illegally. According to family members, he has already refused an offer of a home and a well-paying job in exchange for keeping his mouth shut.

Mr. Hada's case shows how severely Beijing deals with anyone who requests the autonomy supposedly guaranteed to regions like Inner Mongolia in China's constitution. The Mongolian ethnic minority has never posed a political threat on the scale of Tibetans or Xinjiang's Uighurs. That's largely because the government has successfully encouraged ethnic Han migration. Today Inner Mongolia is 80% Han and 20% minorities, as compared to the reverse in 1949.

Mr. Hada's insistence on Mongolian self-determination may seem anachronistic to some. But the severity of Mr. Hada's initial punishment, and the opaqueness surrounding his release, shows that Beijing remains acutely aware of the weakness of its mandate to rule in areas where minorities have been subject to abuses for generations. The fact that a bookseller must be locked up for 15 years and then disappears simply for promoting Mongolian culture suggests that China's vaunted stability is more fragile than it looks.