PORTLAND (AP) — The deaths of six sea lions found in traps on the Columbia River earlier this month were likely caused by the heat, the National Marine Fisheries Service said.

The states of Oregon and Washington had been trapping the animals as part of a federally approved removal process to reduce their impact on salmon at Bonneville Dam.

Federal and state officials initially said the sea lions had been shot, but they did an about-face after necropsies by state and federal experts found no such evidence.

The fisheries service said Wednesday the results of necropsies on all six animals were consistent with death from heat prostration. Studies of tissue samples taken after the May 4 deaths are expected in about 10 days and might reveal more.

The Humane Society of the United States questioned whether the 60-degree weather would kill the animals. Marine mammal experts, however, said the blubbery animals are used to swimming in cold water and can rapidly overheat even when air temperatures are comfortable to humans.

Panic as the sea lions realized they were trapped in the cages could also have caused them to exert themselves, further elevating their body temperatures, Steven Brown, veterinarian for the Oregon Coast Aquarium in Newport, told The Oregonian newspaper. At least one of the sea lions had lacerations believed to be inflicted by another animal, a hint of unrest inside the floating cages.

It’s still not known what caused the doors of the cage traps to close.

They have been known to shut inadvertently, but Brian Gorman, a spokesman for the fisheries service, said it’s unlikely the doors of two cages would accidentally close at the same time.

Four of the dead animals were California sea lions, which are protected by the Marine Mammal Protection Act.

The two others were Steller sea lions, protected by the Endangered Species Act.