Kazakh rights activist Serikzhan Bilash in Almaty, Kazakhstan, on Feb. 13. (Mariya Gordeyeva/Reuters)

THE WAY that China’s leaders tell it, the string of camps they have built in Xinjiang province in the country’s northwest are for vocational education, meant to combat extremism. A recent commentary by the official news service Xinhua declared that China has “significantly improved people’s sense of security and happiness in the autonomous region.” If that is the case, then why is Serikzhan Bilash, an activist who has called attention to mass detentions in Xinjiang, under arrest?

Mr. Bilash, who was born in Xinjiang, is now a naturalized Kazakh citizen. Many detainees in the Chinese camps are ethnic Muslim Uighurs, but a significant additional portion of those arrested are Kazakhs. Mr. Bilash has run an advocacy organization in Almaty, Kazakhstan, to tell their story and call attention to the Xinjiang concentration camps, which witnesses say are designed to forcibly eradicate the culture, language and traditions of the Uighurs, Kazakhs and others. Mr. Bilash organized several gatherings of ethnic Kazakhs from Xinjiang who settled in Kazakhstan and complained that their relatives were held in the camps, according to Radio Free Europe/Radio Liberty.

Mr. Bilash, who has tangled with the Kazakh authorities in recent months over legal registration of his group, was arrested at 2:30 a.m. on Sunday in Almaty by Kazakh authorities and flown to Astana, the capital. The office of his group, Atajurt, was raided by police, who confirmed he was being held on suspicion of inciting “national discord or hatred” but provided no details.

Kazakhstan borders Xinjiang and plays an important role in China’s expansive Belt and Road Initiative to forge new trade routes to Europe and elsewhere. Sixty percent of China’s land transit shipments to Europe now pass through Kazakhstan, according to China’s ambassador to Kazakhstan; the two nations have a major trade relationship with each other, and the growing web of shared transit routes gives Kazakhstan access to faraway ports. It should come as no surprise that Kazakh authorities might heed a request from Beijing. Kazakhstan’s leader, Nursultan Nazarbayev, presides over an authoritarian system at home and has navigated carefully among Russia, China and the West.

In return for its investment, China demands loyalty and leverage. Its rulers have been pummeled with international criticism following disclosure of the vast scope and malevolent purpose of the Xinjiang camps. The work of Mr. Bilash most certainly was nettlesome in Beijing, not in the least because he enabled eyewitnesses to tell the truth about the camps, a truth that China has long denied: that they are a factory of cultural genocide.

Mr. Bilash appeared in a video made by the Kazakh authorities after his arrest in which he said he was not taken “by either the Chinese or Chinese spies.” That’s hardly comforting, knowing how China exercises a long arm to snatch or punish adversaries. Mr. Nazarbayev should not do the bidding of China’s secret police, and Mr. Bilash should be freed immediately. Otherwise, a small but valuable window on the tragedy of Xinjiang will go dark.