BEIJING - The concrete roof of a school gymnasium collapsed in China's Qiqihar city, killing 11 people, state media reported on Monday (July 24), with many victims believed to be young female volleyball players.

Authorities said the illegal stacking of materials on the roof may have caused it to cave in, Xinhua reported.

Initially 15 people were trapped underneath the rubble and state media learned at 10am that authorities had pulled out the last person buried beneath, a student, who showed no vital signs.

The collapse at the No. 34 Middle School in Longsha District in Qiqihar, located in northeast China's Heilongjiang province, was reported at 2.56 pm on Sunday, according to the provincial fire and rescue department, Xinhua said.

A female volleyball team was training in the gymnasium at the time, a man told China Youth Daily as he waited anxiously at the local hospital for news of his 16-year-old daughter.

The students "had just returned to the school a few days ago after a competition out-of-town," an eyewitness told state radio.

It was not immediately clear if any victims were adults, but state radio reported on Sunday that the coach of the team was buried underneath the rubble.

There were 19 people at the gymnasium when the accident occurred, with four managing to escape, said authorities.

In a video that went viral on social media, an angry and distressed father said the government sent police to keep a watch on parents but did not send anyone to update them about their children.

"They tell me my daughter is gone but we never got to see the child. All the children had their faces covered with mud and blood when they were sent to the hospital. I pleaded, please let me identify the child. What if, that wasn't my child?" the man said.

An overhead aerial view of the scene from social media pictures showed a completely collapsed roof with rescue workers in the gym next to large boulders of concrete.

Other pictures showed large cranes hoisted on the side of the school building as rescue efforts continued.

The region and several parts of China had heavy rain this weekend, causing flooding and damage in some areas.

A preliminary investigation found that construction workers illegally placed perlite, a mineral with high water content and which can absorb water, on the roof of the gymnasium during construction of a teaching building adjacent to the gymnasium, Xinhua reported.

Under persistent rains, the perlite soaked up water and gained weight, resulting in the roof collapse, state media said.

The accident was condemned in an editorial by The Beijing News, a Chinese Communist Party-owned newspaper, which criticised the lack of construction supervision.

"Only by truly respecting construction norms, respecting safety, and strengthening the supervision and management of the construction process can we avoid repeating the mistake," it wrote.

An investigation is ongoing and individuals in charge of the construction company have been taken into police custody, Xinhua said.

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Source: Reuters