MADISON, Wis. (AP) — Federal safety inspectors say an air filter blew off a corn grinding device shortly before a deadly explosion demolished a southern Wisconsin corn mill last summer.

Officials with the U.S. Chemical Safety and Hazard Investigation Board held a news conference Monday to discuss preliminary findings from their investigation into the deadly blast at the Didion Milling Plant in Cambria on May 31, 2017. Five workers were killed and 14 were injured.

CSB investigators spoke with 10 of 14 supervisors. A superintendent told them that he saw the filter blow off the grinder. Flames shot out of the intake line. He fled moments before the plant exploded.

CSB Investigator Mary Beth Mulcahy says the board's probe is continuing and it could be six months before it issues any conclusions.