SIDOARJO – At least 91 students remain trapped in concrete rubble almost two days after an Islamic school building collapsed on them, authorities said after reviewing attendance records and reports from anxious families of those missing.

More than 300 rescue workers desperately worked to free survivors Wednesday morning, after the structure fell on top of hundreds of people, mostly teen boys, who had been performing afternoon prayers Monday in a prayer hall at a century-old al Khoziny Islamic boarding school in East Java province that was undergoing an unauthorized expansion.

At least three students have been confirmed dead and 100 others were injured, many with head injuries and broken bones.

The National Disaster Management Agency revised the number of people presumed buried in the rubble to 91 late Tuesday from the previous 38.

The agency said at least six children are alive under the rubble, but the search was made difficult by the slabs of concrete and other unstable, heavy parts of the building. Heavy equipment was available but not being used due to concerns that it could cause further collapse.

Rescuers have been running oxygen, water and food from narrow gaps to those still trapped under the debris to keep them alive, some of them pinned in place.

The students were mostly boys in grades seven to 12, between ages 12 and 18. Female students were praying in another part of the building and managed to escape, survivors said.

The prayer hall had been two stories but two more were being added without a permit, according to authorities. Police said the old building’s foundation was apparently unable to support two floors of concrete and collapsed during the pouring process.