State investigators rule Marengo fire accidental

State fire investigators say the explosion that injured a dozen people at a Marengo biofuels plant was an accident. The State Fire Marshal's Office says an "unknown mechanical failure" ignited flammable vapor at the C6 Zero plant last December.Investigators say the company was dissolving roof shingles with a chemical at the time.The Iowa DNR and the Iowa Attorney General's Office have been locked in a legal battle with the company to learn what those chemicals are and to clean up the contaminated site.