AP

Police and investigators from Oregon Fish and Wildlife investigate the death of sealions held in this cage at the Bonneville Dam on the Columbia River between Washington and Oregon, east of Portland.

PORTLAND, Ore. – After reviewing necropsy findings for six sea lions found dead at the Columbia River this month, investigators say the animals died from heat exhaustion.

Four California sea lions and two Stellar sea lions were found May 4 near the Bonneville Dam, where Washington and Oregon were trapping the animals to keep them from eating salmon.

Authorities first believed the sea lions were shot to death, but necropsy reports found no evidence of recent gunshot wounds.

On Wednesday, NOAA's National Marine Fisheries Service said the sea lions had died of heat prostration.

Officials say more conclusive results should be available in another 10 days, once tissue sample studies are completed.

The investigation into the deceased sea lions remains open.