California authorities are investigating the fatal shooting of a protected southern sea otter that had been rescued and returned to the ocean last year.

The state Department of Fish and Wildlife said Tuesday that the dead otter was found floating in Morro Bay in September and a necropsy determined it was killed with a pellet gun.

A numbered tag identified the animal as an otter that had previously been found tangled in a fishing line. It was rehabilitated and returned to the wild in July 2016.

Southern sea otters, which are also known as California sea otters, are listed as a threatened species under the federal Endangered Species Act, are considered depleted under the Marine Mammal Protection Act, and are protected by California state law, according to the Department of Fish and Wildlife.

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The department says the killing was the latest in a string of sea otter deaths along the stretch of coastline between San Francisco and Los Angeles.

A skinned otter was found in September on Arroyo de la Cruz Beach in San Simeon. The otter was a young adult female, according to initial necropsy results.

Three otters were shot to death in the Santa Cruz area in August 2016.

SHARE COPY LINK A 9-year-old sea otter named Otto that was sickened by domoic acid was released back into Morro Bay by the Marine Mammal Center on Friday, September 22, 2017.

Killing an otter is punishable by up to $100,000 in fines and possible jail time. Removal and possession of an otter's pelt without a permit is also illegal, even if the sea otter was already dead when it was found, officials said. Otters, whose range once reached outside of California, are now found from Santa Barbara to San Mateo counties.

Officials ask anyone with information about this killing or other otter killings to contact the state Department of Fish and Wildlife at 888-334-2258 or call a special agent from the U.S. Fish and Wildlife Service at 916-569-8444.

Authorities also remind people that anyone who finds a dead or injured sea otter in California should leave it where it is, take a photo if possible and report it to the state Department of Fish and Wildlife at 805-772-1135.

The Associated Press contributed to this story.