An animal rescue organization is offering a reward for information after finding a dolphin shot to death in Manhattan Beach, California.

Marine Animal Rescue, of El Segundo, posted a $5,000 reward to their Facebook page on Friday after a necropsy revealed a dolphin died from a gunshot wound.

Peter Wallerstein, founder and president of the rescue group, received a call about the dolphin in the surf on the morning of Nov. 8 and managed to retrieve the animal with the help of a lifeguard.

"It's just a vicious act of brutality," Wallerstein told NBC News on Friday.

A doctor with Marine Mammal Care Center Los Angeles, a local rehab facility, was able to retrieve the bullet that killed the dolphin, Wallerstein said.

Wallerstein said he contacted the National Marine Fisheries Service to open an investigation into the incident.

"They’ll do the best they can," Wallerstein said. "They have the bullet and they have evidence of the shooting. We very rarely find these people but what the reward does is put them on notice."

Wallerstein has been working in animal rescue for more than 30 years and says he cannot fathom a reason someone would shoot a dolphin. And though he's never seen an instance like this one before, Wallerstein told NBC News that it could be the sign a bigger problem.

"I've had sea lions shot but never had a dolphin," he said. "But maybe the dolphins have drifted back into ocean, we don't know."