YINCHUAN, Sept. 28 -- An initial investigation showed that the deadly coal mine explosion in northwest China's Ningxia Hui Autonomous Region on Tuesday was caused by illegal mining, according to a press conference held early Wednesday.

The explosion occurred at 7 a.m. Tuesday at a small coal mine, which belongs to Linli Coal Mining Co. Ltd. in Shizuishan City, when 20 miners were working under ground.

The people who are responsible for the company have been under police control, according to the conference.

So far, 18 miners have been confirmed dead while two others remain missing. Rescue work continues.

Wu Yuguo, vice mayor of Shizuishan, said at the conference excessive gas concentration and destroyed mine shaft have made the rescue more difficult. The government has set up a working group to deal with the aftermath and comfort the miners' families.

Gui Laibao, deputy chief of the State Administration of Coal Mine Safety, has arrived at the site of explosion to inspect rescue operations.

The mine's annual production capacity is 450,000 tonnes. Its certificate remains valid until September 2017.

Most of the miners were from the provinces of Sichuan and Gansu, as well as Ningxia.