Two dolphins were found dead with similar injuries within one week along Florida's gulf coast, prompting investigators to offer a reward of $54,000 (£42,000) for information.
Biologists with the Florida Fish and Wildlife Conservation Commission (FWC) discovered a dead dolphin off Naples in southwest Florida last month. The porpoise had been killed by a bullet or sharp object, investigators said.
The same week, Emerald Coast Wildlife Refuge experts recovered a dolphin along Pensacola Beach in the Florida Panhandle. A bullet was found lodged in the side of the marine mammal.
The National Oceanic and Atmospheric Administration (NOAA) first offered a $20,000 (£15,600) reward last month before raising the amount to $54,000 on Monday in combination with money pledged by animal rights groups.
Experts believe the two deaths might have stemmed from humans feeding the animals. When dolphins learn to associate people and boats with food, they can expose themselves to dangerous situations.
The NOAA urges people to not feed wild dolphins.
It is a crime in the US to harass, hunt or feed wild dolphins under the Marine Mammal Protection Act. Offences can be punished by up to $100,000 (£78,000) in fines and up to one year in prison.
Since 2002, at least 29 dolphins have washed up on the southeast US coast with evidence of being shot by guns or arrows, or impaled with objects such as fishing spears. In May 2019, a dolphin was found dead off Captiva Island with a fatal puncture wound to its head. An investigation into that death is still ongoing.
Additional reporting by Associated Press
