The Tuesday explosion of a wood pellet manufacturing plant in Fremont that killed a worker and two young girls was caused by a dust fire, an official said Thursday.

The scene is at Horizon Biofuels Inc., located near East Cloverly Road and South Union Street. Fremont Mayor Joey Spellerberg said three people were inside the plant when it exploded just before noon Tuesday. He said the three people are missing and unaccounted for at this time.