ST. CHARLES — Investigators have been unable to determine what caused a St. Charles motel fire that killed a South Dakota man in December.

The State Fire Marshal’s office has classified the cause of the fire as undetermined. The file was closed Friday.

Police said 47-year-old Vincent Edward Wright, of Avon, S.D., had checked into the Downtown Motel on Dec. 6 and was the only one registered to the room. Three other motel guests escaped unharmed when fire broke out about 11 p.m.

Investigators ruled out electrical causes and found no ignitable liquids.

The medical examiner found Wright died of carbon monoxide poisoning and thermal injuries. His body had no signs of foul play.

The file may be reopened if authorities get new information.