The concrete roof of a school gymnasium in China's Qiqihar collapsed, killing eleven people, believed to be mostly young female volleyball players. Authorities believe that the illegal stacking of materials on the roof could be the cause. 

The collapse occurred at the No. 34 Middle School in Longsha District in Qiqihar, located in the northeast Heilongjiang province. 

There were 19 people at the gymnasium when the accident occurred on Sunday afternoon, with four managing to escape. The last remaining person wasn’t pulled out of the rubble until Monday morning, authorities said.

A father told China Youth Daily that a female volleyball team was training in the gymnasium when the incident occurred, and an eye-witness told state radio that “The team consisted of students selected from different grades. They had just returned to the school a few days ago after a competition out-of-town."

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

A preliminary investigation found that construction workers were illegally placing perlite, a mineral with high water content and which can absorb water, on the roof of the gymnasium during the 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.

Individuals in charge of the construction company have been taken into police custody and an in-depth investigation is ongoing, Xinhua said.