The student's friends think he joined the thousands — possibly tens of thousands — of people, rights groups and academics estimate, who have been spirited without trial into secretive detention camps for alleged political crimes that range from having extremist thoughts to merely traveling or studying abroad. The mass disappearances, beginning the past year, are part of a sweeping effort by Chinese authorities to use detentions and data-driven surveillance to impose a digital police state in the region of Xinjiang and over its Uighurs, a 10-million strong, Turkic-speaking Muslim minority that China says has been influenced by Islamic extremism.