Authorities believe the school in Sumba Island town of Bima was actually a militant training camp, National Police spokesman Brig. Gen. Ketut Untung Yoga said. Immediately after Monday's blast, school officials and students stood guard in front of the building, preventing security forces from entering.

At the end of the 48-hour standoff, police discovered explosives, dozens of machetes, air rifles, Molotov cocktails and jihadi books and videos. The headmaster, Abrori Ali, was arrested Friday in his parents' house, he said, and seven other people are in police custody for questioning.