BEIJING—China’s mass detention campaign against Muslims in its far west is targeting people who have moved overseas, with Chinese authorities investigating whether members of the Uighur ethnic group are involved in any antigovernment activities.

For more than a year, security officials have told Uighurs living abroad to provide documentation of their overseas activities and to spy and inform on other Uighurs, most of whom are Muslim, according to interviews and chat records reviewed by The Wall Street Journal. Other expatriate Uighurs have had their passport renewals denied and instead were offered one-way travel documents to try to force them to return to China.

In many cases, these Uighurs said, authorities are leveraging the government’s detention program, threatening to throw family members still in Xinjiang into the centers, where the U.S. State Department estimates hundreds of thousands have been detained.

Namtulla Najmidin, a 35-year-old computer-engineering student in Norway, said his father called in May last year telling him to expect to hear from the local police. A man identified as a police officer soon followed up, asking for pictures of his passport and Norwegian identification documents.

“Those are my private things. I can’t give it to you,” Mr. Najmidin wrote back on May 18 last year, according to chat logs reviewed by the Journal.