The explosion happened at about 7:00 a.m. in the Hegang office building of Heilongjiang Sheng'an Civil Explosive Material Ltd., killing a security guard and an unidentified man.

Nearly all the windows in the four-storey building were shattered by the explosion. The building was badly damaged and too dangerous for investigators to enter, local police said.

Architectural experts have contracted to offer advice.

Police are still investigating the cause of the accident.