An investigator in western Illinois says four adults whose bodies were found on a Mississippi River houseboat may have been victims of carbon-monoxide poisoning.

Police say the bodies of two men and two women were found Sunday afternoon on a boat just north of the village of Hamburg.

Mark Wagner of the Illinois Department of Natural Resources' conservation police says the group had been camping and the houseboat was tied to an island.

Autopsies are planned.

Hamburg is about 50 miles northwest of St. Louis.