This aerial photo taken on July 24, 2023 shows the site of a collapsed middle school gymnasium in Qiqihar, Northeast China's Heilongjiang province. Eleven people have been killed in roof collapse of the middle school gymnasium that took place on Sunday in Qiqihar, rescuers said Monday. (Photo/Xinhua)

Eleven people were confirmed dead after the ceiling of a gymnasium collapsed on Sunday afternoon at a school in Qiqihar, Heilongjiang province, local authorities said on Monday. An investigation is underway.

The accident, which occurred at the gymnasium of No 34 Middle School in the city's Longsha district, trapped 15 people, Xinhua News Agency reported. Four others had managed to escape following the collapse.

After receiving an emergency call at 2:56 pm on Sunday, the Qiqihar fire rescue command center immediately dispatched a team to the scene, including 39 firetrucks, 159 firefighters and four sniffer dogs, the National Fire and Rescue Administration said on its WeChat account. The center also notified medical, public security, emergency, electric power and other related departments to coordinate work at the scene.

At 3:10 pm, the rescue team arrived and said the roof of the gymnasium had completely collapsed. As of Monday morning, all 15 trapped people had been located, among whom five had no vital signs when found and six others died in the hospital. The four others had non-life-threatening injuries.

According to a report released by Hubei province-based Jimu News, the 15 trapped people had included two coaches. A female coach surnamed Ding was confirmed dead, and a male coach surnamed Gao was being treated at a hospital for a foot injury. The gymnasium building is around 1,200 square meters with a steel frame; the roof was composed of a concrete slab.

A preliminary investigation found that construction workers had illegally stacked perlite, an insulation material, on the roof of the gymnasium during construction of a teaching building next to the gym. Due to recent rainfall, the perlite had soaked up water, which increased its weight and resulted in the roof's collapse.

Those responsible have been taken into police custody.

Perlite "is often used in external wall insulation", a construction industry staff member surnamed Chen told Shanghai-based Xinmin Weekly. "It will gain more than 10 times its weight after absorbing water."

"With quite a large area, gymnasiums seldom use pillars in the middle to support the roof, so the roof's load-bearing capacity is very low," said Chen. "The roof is like a huge tent, and the stacking of building materials on it should not have been allowed."

The father of a trapped student told Beijing Daily that his 16-year-old daughter and other members of the school volleyball team had been training at the school for an upcoming competition in Hubei province. It was not immediately known if his daughter was among the survivors.

On Sunday night, the provincial government held a video conference on safety work in Heilongjiang to provide information on the roof collapse and other recent accidents.

These accidents "exposed serious problems", said Heilongjiang Vice-Governor Wang Yixin. "It is necessary to carry out special rectification of safety risks, strengthen management of any dangerous buildings" and take steps to ensure the safety, he said.