YINCHUAN -- Four people have been put under criminal detention for allegedly "negligently causing a serious accident," after a fatal explosion at a barbecue restaurant in Northwest China's Ningxia Hui autonomous region, said local authorities on Saturday.

The explosion occurred at approximately 8:40 pm on Wednesday at Fuyang Barbecue Restaurant on a busy street in Xingqing district of Yinchuan, the regional capital, when liquefied petroleum gas leaked from the restaurant. The explosion has killed 31 people and left seven injured. All the injured are now in stable condition after receiving medical treatment.

The restaurant operator surnamed Ma, two shareholders surnamed Zhang and Liu, and the restaurant manager surnamed Suo have been detained. The four are among the nine people previously put under control by public security authorities. Others involved in the tragedy are still under investigation.