Photo:Weibo account of People's Daily

Eleven people have been killed in a roof collapse in a middle school gymnasium that took place on Sunday in Qiqihar City, northeast China's Heilongjiang Province, rescuers said Monday.

All 15 trapped were pulled out as the last trapped person was found Monday morning without vital signs, the rescue headquarters for the accident said at around 10 a.m. Monday.

The remaining four people have been checked without life-threatening injuries.

The roof collapse was reported at 2:56 p.m. Sunday, according to the provincial fire and rescue department. There were 19 people in the gymnasium covering about 1,200 square meters at the No. 34 Middle School of Qiqihar in Longsha District when the accident occurred. Four people escaped on their own and 15 were trapped, the municipal search and rescue headquarters said.

A preliminary investigation found that construction workers illegally placed perlite on the roof of the gymnasium during the construction of a teaching building adjacent to the gymnasium. Under the influence of rainfall, the perlite soaked up water and gained weight, resulting in the roof collapse.

An in-depth investigation is ongoing. Those in charge of the construction company have been taken into police custody.