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...