An ethanol plant in Glenville, Minn., remained closed Monday after an explosion early Sunday blasted metal parts through the exterior wall and set off sprinklers inside.

Two men working in the Poet plant were injured, but they managed to walk out of the building after the blast and their injuries were not considered life-threatening, plant and local officials said.

The State Fire Marshal's Office was investigating the blast, which happened at 3:17 a.m. Sunday in an area that dries a byproduct animal feed. It was not immediately clear whether ethanol, natural gas used in the plant or something else blew up.

Matt Merritt, a spokesman for Sioux Falls, S.D.-based Poet, said it also was not known how long the plant would be closed. He said the plant typically employs about 50 people, but he didn't have the exact employment figure on Monday. No layoffs are planned, he said.

After the explosion, a large elbow section of steel pipe was found outside, said Freeborn County Sheriff Kurt Freitag. "There were holes punched through the building from debris being blasted out," he said.

The ethanol plant is managed and part owned by Poet, the nation's second-largest ethanol producer. Local farmers also are part owners. An adjacent biodiesel plant owned by another company was not damaged, Freitag said.