Detonators caused Friday's explosion at a office building of a mining company in north China's Shanxi Province, police said Sunday.

The illegally bought detonators were stored in the first floor office of a deputy boss of Dongfang Zhenxing Coal Mining Co. Ltd in Nanlou Town, Yuxian County, the Shanxi Public Security Department said.

The blast occurred at 4:30 p.m. Friday in the three-floor office building of Dongfang Zhenxing. Four people, all mine staff, including the deputy boss, died.

The explosion was still being investigated.

(Xinhua News Agency March 22, 2009)