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.

Last week the Humane Society of the United States, which is suing to block the authorized killing or removal of up to 85 animals a year for five years, agreed with the federal government and the states of Oregon and Washington to continue a ban on killing and stop permanent removal until next year, in part to allow more efforts to go toward investigating what was thought to be shootings of the animals over the weekend.

The agreement allowed the governments to continue removing animals and branding them for identification if they return them to their natural habitat.

Trapped sea lions identified as troublemakers at the dam where they gather each spring to eat migrating salmon were being sent to aquariums such as Sea World.

Arguments were to have been heard before a panel of the 9th U.S. Circuit Court of Appeals on Thursday.

Under the agreement a hearing on the sea lions would be expedited but killing or permanent removal could not take place until then, probably early next year.