Authorities are trying to determine what caused an explosion that injured five people performing electrical work at a manufacturing plant in northwest Iowa.

The Osceola County Sheriff’s office says the explosion happened at the Timewell Tile plant in Sibley, Iowa, Saturday morning while workers were switching some power lines to a newer transformer.

Lt. Seth Hoffman says five men sustained burns in the accident. Some of the burns were extensive, but authorities didn’t immediately release their names so their current condition wasn’t available.

Hoffman says two of the injured workers were employed by Timewell. The other three were employed by Current Electric. Two city workers outside the plant were unhurt.