PORTLAND, Ore. -- Six sea lions found dead in closed traps at Bonneville Dam earlier this month died from heat prostration, officials said Wednesday.

Brian Gorman, regional spokesman for the National Marine Fisheries Service, said the sea lions died from overheating that caused organ failure. Heat prostration tends to be caused by vigorous exercise.

One sea lion that had a puncture wound was likely attacked by another sea lion, Gorman said.

The sea lions were initially believed to have died from gunshot wounds, triggering a multi-agency investigation. But three days after the carcasses were found, officials from the National Oceanic and Atmospheric Administration determined a shooting wasn't the animals' cause of death.

Fish and wildlife officials checked on the sea lions May 4 after noticing the trapping gates were closed. The traps, authorities said, were supposed to remain open.

More conclusive necropsy results will be available once studies of tissue samples taken from the dead animals are completed.