An investigation is underway along the Central California coast in connection with the shooting deaths of three southern sea otters at Asilomar State Beach in Monterey County.

The US Fish and Wildlife Service announced Friday that a $21,000 reward is being offered for information that leads to the arrest and conviction of the person or persons responsible for killing the sea otters.

The otters were killed in September 2013, but no information regarding their deaths was released until Friday. The USFWS said necropsies were completed on the three otters, two of which were shot in the head and the third was shot in the back. Lead-coated bullets were determined to be the cause of death.

"With all of the natural threats and human-caused problems facing sea otters, it's especially tragic that a person would set out to intentionally kill these sea otters," Andrew Johnson, Sea Otter Research and Conservation Program Manager for the Monterey Bay Aquarium, told local news station KSBW.

Southern sea otters, also known as the California sea otter, were listed as threatened in 1977 under the US Endangered Species Act. They are also protected by California law. Killing a California sea otter is punishable by a fine of up to $100,000 and possible jail time, the USFWS said in a statement.

California Department of Fish and Wildlife Agent Rebecca Roca told the Monterey County Weekly newspaper that the reward, which is financed by a host of public and private entities and concerned citizens, was intended to give the investigation a boost.

"We've talked to a lot of people, we've got a lot of information," Roca said. "We're stepping up right now so that we can find these people and prosecute them."

According to the USFWS:

Anyone with information about the sea otter shootings should contact Special Agent Souphanya of the US Fish and Wildlife Service at 650-876-9078. An anonymous report can also be made by calling the US Fish and Wildlife contact line at 703-358-1949, or the California Department of Fish and Wildlife CalTIP line at 1-888-DFG-CALTIP.