Eleven people died after the roof of a middle school gymnasium collapsed on Sunday in Qiqihar City, northeast China's Heilongjiang Province, local authorities said Monday as the rescue operations concluded.

The roof collapse was reported at 2:56 p.m. on Sunday, according to the provincial fire and rescue department. 

When the incident took place, there were 19 people inside the 1,200-square-meter gymnasium at the No. 34 Middle School in Longsha District. Four of them narrowly escaped as the roof caved in while the rubble trapped 15, of which four survived.

According to a preliminary investigation, construction workers had illegally placed perlite on the gymnasium roof during the construction of an adjacent building. The perlite soaked in water after rain and gained weight, causing the roof to cave in. The gymnasium's walls have a grid structure, and the roof is made of concrete slabs, according to local authorities.

An in-depth investigation is underway to ascertain the cause of the incident while some people in charge of the construction company have been taken into police custody, the authorities said.

(Cover: Rescuers search for people trapped after the roof of a school gymnasium collapsed in Qiqihar City, northeast China's Heilongjiang Province, July 23, 2023. /CMG)