BEIJING (AP) — A natural gas explosion at a shopping mall in southwestern China killed four people and injured 35 others, local authorities said Friday.

Three floors of the Moerma shopping center in the city of Luzhou in Sichuan province were engulfed in fire after the explosion late Thursday night, the local government said in an online statement.

The local work safety agency said four people died and 35 were injured — six of whom were in serious condition.

Local news reports said a gas leak was reported and that about 200 people inside the mall were evacuated before the explosion shook the building.

A moviegoer, identified by the family name of Xiao, told the local newspaper that he had initially mistaken the tremor as special effects. "Then, we saw dust falling inside the theater and thought it was an earthquake," Xiao said.

By morning, local authorities had cordoned off the area and urged residents not to gather to look at the scorched structure. News photos showed shattered glasses and blackened wall panels.

Local authorities said the cause of the explosion was under investigation.

Workplace safety is a major problem in China, where safety regulations are often ignored.