Eighteen people have been killed in a gas explosion in a small coal mine in northwest China’s Ningxia Hui Autonomous Region, state-owned news portal China News reported Wednesday.

The explosion at Linli Coal Mining Co. Ltd. in Shizuishan City occurred at 7 a.m. on Tuesday. In a press conference on Wednesday morning, city authorities said the blast was caused by illegal mining, and that the owner of the enterprise has been taken into custody by local police.

At the time of the accident, 20 people were reportedly trapped underground. Eighteen bodies have been found, and two people are still being searched for. Rescue operations are hindered by lingering gases and structural damage to the mine, Wu Yuguo, vice mayor of Shizuishan, said at the press conference.

According to state news agency Xinhua, the mine’s annual production capacity is 450,000 tons of coal. Its current mining certificate is valid until September next year.

Linli Coal Mining Co. Ltd. could not immediately be reached for comment by Sixth Tone on Wednesday.

Explosions frequently rock industrial and mining operations in China. Greenpeace East Asia said on Sept. 21 that the country had seen an average of 29 chemical accidents per month in 2016, 16 percent of which were explosions. In August, 21 people were killed in an explosion at an energy plant in the city of Dangyang, in central China’s Hubei province.

(Header image: The mining site at Linli Coal Mining Co. Ltd. in Shizuishan, Ningxia Hui Autonomous Region, Sept. 27, 2016. @yangshixinwen from Weibo)