The Prakasam Police unravelled the mystery behind the death of a 15-year-old Dalit schoolboy, Keshav Kumar, with the arrest of three persons at Ponnalur in Kandukur division on Wednesday. The arrested are 22-year-old Sk. Alla Baksh, Sk. Mastan Vali and Balakrishna (20). Kandukur Circle Inspector M. Lakshman said the SC/ST prevention of Atrocities Act as well as IPC sections 302 (murder) and 201 (suppression of evidence) had been invoked against the accused.