PORTLAND — Federal officials investigating the deaths of six sea lions at a Columbia River dam did an about-face Wednesday, saying their initial assumption that the animals had been shot to death was wrong.

Officials have not ruled out human involvement, but the conclusion from preliminary necropsy results that shootings did not kill them reopened questions of how the animals died.

The National Marine Fisheries Service’s initial reports about the deaths raised intense interest in the long-running dispute over the sea lions, which prey on protected salmon. As a consequence, the government and the Humane Society of the United States agreed to suspend trapping and removing the sea lions this year.

But the preliminary results of the examination of the carcasses found no evidence of recent gunshot wounds, fisheries spokesman Brian Gorman said.

The necropsy results showed shallow puncture wounds in one animal consistent with sea lion bite marks, and X-rays found metal fragments in soft tissue near the neck of two animals, Gorman said.

A metal slug was found in the blubber of one animal. But neither the fragments nor the slug appear to be fatal and may have been from old wounds, he said.

Mark Oswell, a wildlife enforcement officer for the service in Silver Spring, Md., said Wednesday that human involvement still cannot be ruled out and said dehydration, heat exhaustion or panic could have been factors. It still is not known what caused the doors of the cage traps to close, he said.

Toxicology tests will be done if further investigations warrant it, and tissue samples have been taken, he said.

The sea lions’ carcasses have been taken to the federal fish and wildlife forensics laboratory in Ashland, he said. Officials there declined to comment.

The service reported Sunday that the animals had apparently been shot and said later that investigators were pursuing theories that somebody in a boat had gone to the traps and used a high-powered rifle to dispatch the sea lions, the bullets passing through the flesh.

A spokesman for the Oregon Department of Fish and Wildlife, Rick Hargrave, said Monday that the sea lion cages had been checked about 7 p.m. Saturday and again at noon on Sunday, when a student intern reported that the drop-down cage doors were closed. An Oregon state employee then confirmed the deaths. Two cages held three sea lions apiece, Gorman said Sunday.

On Wednesday, a Human Society official blamed “an obvious security lapse” in the trap area, which was closed to the public.

“Those animals did not close the gates by themselves and die,” said Sharon Young, the organization’s field director for marine issues. “Somebody had a hand in killing them. What that person did, nobody knows.”