The roof of a school in northwestern Pakistan has collapsed, killing at least 34 students and a teacher and injuring more than 50 others, authorities and a hospital official said.

The incident happened on Thursday at Al-Khair Education Centre, a private school in Peshawar, the capital of Khyber Pakhtunkhwa province.

Mohammad Asim, spokesman for Lady Reading Hospital, where many victims were transported, confirmed the casualties, saying some of the injured were in critical condition. Most of the children killed or injured were between six and 12 years old, and the oldest was 14, he said. The hospital has declared an emergency as medical staff treat the wounded.

Hours after the collapse, rescue officials said everyone trapped under the rubble had been recovered. An emergency services spokesperson, Bilal Faizi, said that nearly 100 rescuers helped the operation, including parents who helped the crew.

He credited the rapid evacuation with saving at least 40 lives. More than a dozen children suffered minor injuries, he said.

Peshawar’s police chief Saeed Ahmed said that the owner of the school has been arrested. He said the building was structurally weak.

However, following his arrest, the owner, Mohammad Khan, blamed this week’s rains for weakening one of the roofs, according to police, who said the investigation was still under way.

The provincial government said a committee had been formed to investigate the cause of the roof collapse of the three-storey building.

In Islamabad, President Asif Ali Zardari and Prime Minister Shehbaz Sharif expressed sorrow over the deaths and offered condolences to the bereaved families in separate statements.

“The entire nation shares the grief of the affected families in this hour of sorrow,” Zardari said.