BEIJING (Reuters) -The concrete roof of a school gymnasium collapsed in China's Qiqihar city on Sunday killing 11 people, many believed to be young female volleyball players, with authorities saying the illegal stacking of materials on the roof may have caused the cave in.

Initially 15 people were trapped underneath the rubble and state media learned at 10 a.m. (0200 GMT) that authorities had pulled out the last remaining person, a student, who had 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 p.m. (0656 GMT) on Sunday, according to the provincial fire and rescue department, Xinhua said.

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

"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," an eye-witness told state radio.

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

A family member told reporters at a state-backed media outlet that his niece was a member of the school's female volleyball team and was training at the gym at the time of the incident. 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 father complained that the government sent police to keep a watch on parents but did not send anyone to update 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 were still ongoing.

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.

A user on Weibo, China's popular social media platform, said in comments, "How can the relevant department not understand the common sense that you cannot put heavy objects on the roof? This should be thoroughly investigated." An in-depth investigation is ongoing and individuals in charge of the construction company have been taken into police custody, Xinhua said.

(Reporting by Beijing and Shanghai newsrooms; Writing by Bernard Orr; Editing by Michael Perry)