PORTLAND — An examination of the heads of six sea lions who died in traps at Bonneville Dam on the Columbia River hasn’t given federal investigators reason to pursue toxicology tests to see whether the animals were poisoned, an official of the National Marine Fisheries Service said Thursday.

Beyond that, however, federal officials revealed little about the mystery surrounding the deaths of the six animals trapped in cages as part of an effort to keep them from feeding on endangered salmon.

Fisheries officials have ruled out their initial assumption the six sea lions were shot to death and say they are still puzzled about how the animals came to be trapped in the cages. The trap doors had been left open, so the sea lions would grow accustomed to them until it was time to send them to aquariums.

Specialists at a U.S. Fish and Wildlife Service forensic lab in Ashland examined the sea lion heads and sent findings on to the fisheries agency, which is investigating the deaths. Their report wasn’t made public.

“We received the heads. We examined them as best we could and reported the results back to the agents. We are done with them,” lab director Ken Goddard said Thursday. “I think we’ve answered all their questions.”

Spokesman Mark Oswell of NOAA Fisheries said Thursday that based on information so far “there is nothing to indicate a need to do toxicology tests,” which could determine if poison was involved.

Tissue samples have been kept in case such tests are needed later, said Oswell, speaking from Silver Spring, Md.

The sea lions were found Sunday in two traps. Each held three animals, and the doors to both were somehow closed.

The gates of the two traps operate independently of one another, said fisheries service spokesman Brian Gorman in Seattle.

It takes a stout pull on a rope to activate a pin or latch to close the gates if they are working properly, he said Thursday.

“The conundrum for us is how they both closed,” he said, adding that investigators are ruling out nothing.

The trapping program is intended to catch sea lions previously identified by branding or other markings and known to be frequent feeders, or “troublemakers,” at the dam.

Only one of the six found dead Sunday was on the removal list.

For the past several years the sea lions have gathered at the dam’s fish ladders each spring to prey on chinook salmon heading upriver to spawn. The fish are protected by the Endangered Species Act.

So far six have been sent to SeaWorld facilities in Texas and Florida. A seventh died under anesthesia during a health check at a holding facility near Tacoma, Wash.

At 1,452 pounds, the animal that died was one of the biggest California sea lions ever recorded and apparently died of its own weight bearing down on its internal organs, the Washington Fish and Wildlife Department reported.

Gorman said size does count among the sea lions. Especially large sea lions can more easily defend their harems in the breeding grounds to the south, where they go after the salmon run drops off, usually by early June.

He said they tend not to eat during that time, and “a big fat reserve is an advantage.”