MARSHALLTOWN — Authorities are investigating what caused a methane-fueled fire at the JBS pork plant in Marshalltown. Firetrucks were dispatched to the plant’s methane collection lagoon about 10:30 a.m. Saturday. Firefighters and JBS staffers isolated the plant from the methane collection system to prevent any gas exposure to the plant itself. Officials say it took about three hours to extinguish the blaze. No injuries have been reported. JBS officials estimate damage at $500,000.