PORTLAND, Ore. -- The six sea lions initially thought to have been shot to death in traps near Bonneville Dam actually died of heat exhaustion, officials said Wednesday.

A statement from the National Oceanic and Atmospheric Administration said the determination was made after reviewing necropsy findings on four of the sea lions that were found dead May 4th.

Investigators hope studies of tissue samples will be able to provide more conclusive information.

Officials initially suspected the animals were shot after finding puncture marks on the animals' flesh, and because the first examination of the carcasses turned up no slugs, investigators had believed the animals were shot at close range with high-powered rifles, the bullets passing through the flesh.

X-rays turned up metal fragments near the necks of two of the sea lions, and a metal slug was found in the blubber of another.

But NOAA fisheries spokesman Brian Gorman said neither the fragments nor the slug appear to be fatal and may have been from old wounds, he said.

The sea lions included two endangered Steller sea lions and one California sea lion pup.

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