ROCKTON, Ill. (AP) — A senior fire official says an explosion and massive fire at a chemical plant in northern Illinois that filled the sky with black smoke and forced the evacuation of hundreds of homes for days was started accidentally during maintenance work. Rockton Fire Chief Kirk Wilson said in a press release Friday that the June 13 fire at the Chemtool plant near Rockton started when contractors were replacing insulation on heating pipes. Wilson says it’s likely that a scissor lift struck a valve or another piece of piping, releasing oil. He says the workers were trying to contain that leak and depressurize the heating pipes when the fire started. The investigation continues.