BEIJING — Chinese authorities blamed illegal mining activities for a gas explosion at a coal mine that killed 18 miners left two others missing, Chinese state media reported Wednesday.

Tuesday morning’s explosion occurred at a small coal mine when 20 miners were working underground in the city of Shizuishan in the northwestern region of Ningxia, the official Xinhua News Agency said. State broadcaster CCTV had said in the morning that the blast had killed 19 people, but later reported 18 deaths.

Local officials said at a news conference early Wednesday that representatives of the company that owned the mine, the Linli Coal Mining Co. Ltd., were in police custody, Xinhua said.

Xinhua cited an initial investigation as showing that the blast was caused by illegal mining, but did not provide details. The company could not immediately be reached at its listed phone number, which rang busy.

At the briefing, Wu Yuguo, the city’s vice mayor, said an excessive concentration of gas and the destruction of the mine shaft have hindered rescue efforts.

China’s mines have long been the world’s deadliest, but safety improvements have reduced deaths in recent years.

Calls to the Shizuishan city government Wednesday rang unanswered.