Explosions that injured nearly 100 people during Friday prayers could have been an attack, officials said, with a 17-year-old student the suspected perpetrator.

“Several pieces of supporting evidence were found,” police chief Listyo Sigit Prabowo told a press conference after visiting the victims at a hospital.

“There were written materials and some powder that could have potentially caused an explosion,” he said. “We are gathering other records, including examining social media and family members to gather all the information.”

The suspected perpetrator, a student at a school next to the mosque, was recovering after undergoing surgery on Friday, Listyo said.

“The suspect’s condition is improving, and hopefully this will make things easier for us when needed,” he said.