A Wisconsin corn mill that exploded late Wednesday night was reprimanded by federal safety inspectors six years ago for not taking precautions against dust explosions, a major hazard in handling grain, records show. Firefighters also had responded to a fire at the facility two days before the blast, although it’s too soon to know whether the two incidents are related. The explosion at the Didion Milling Plant in Cambria, about 45 miles northeast of Madison, leveled much of the sprawling facility and left two workers dead and a third still missing Thursday night.