An aerial view shows the site where an explosion occured at Mitsubishi Materials Corp's Yokkaichi plant in Yokkaichi, in this photo taken by Kyodo Jan 9, 2014. Five people died in an explosion at a Mitsubishi Materials Corp chemicals plant on Thursday, national broadcaster NHK reported, citing local police. [Photo/Agencies]

TOKYO - An explosion at a chemical factory in Japan on Thursday killed at least five workers and injured 17 others, authorities said.

Investigators suspect chemical reaction involving hydrogen caused the blast at metal and chemical company Mitsubishi Materials Corp.'s Yokkaichi plant, about 350 kilometers (220 miles) west of Tokyo.

The blast appeared to have occurred when plant workers were rinsing heat exchange equipment at the silicone plant during maintenance, according to a Mie prefectural police official, who spoke on condition of anonymity due to department rules.

Police said five workers died. Fire department officials said that 17 people were taken to hospitals.  

The dead and injured were all believed to be workers at the factory.