Two different guns were used to shoot a leopard seal at point blank range on a Northland beach.
That means two people were involved or, less likely, if the first shot failed to kill the animal, the shooter then used a second gun.
Either way, ''it was a cold-blooded execution of a defenceless animal,'' Dr Ingrid Visser, co-founder of Leopardseals.org, said.
''This rules out any claims that this was some sort of mercy killing."
The leopard seal was found dead last Friday, lying where it had been shot near Glinks Gully, a tiny settlement on the coast about 25km south of Dargaville.
Only the day before its presence had enthralled local people who have seen few of that cold-water, solitary species, although other seals are common on the coast.
The chilling results of X-rays, scans and the necropsy (animal autopsy) show multiple shotgun pellets embedded in the head and skull, and wounds in the upper body caused by a rifle.
Pathologist Dr Stuart Hunter, who led the necropsy at the WildBase Vet Clinic at Massey University, said the seal was shot first in the body and then in the head.
"One bullet punctured a lung and broke a rib. The seal would most likely have died in agony,'' Visser said.
"The shotgun blast literally blew the seal's skull apart. The necropsy shows it was shot at point blank range near one of its eyes. There were bone fragments all through its head.''
Images from a CT scan to assess the damage were distorted because there was so much metal in the head.
The young adult female leopard seal was in good health at the time of death, and would have been ''just chilling out on the beach,'' Visser said.
The Department of Conservation (DoC) and Northland Police are investigating the crime.
Police are following a good line of inquiry but anybody with information should still contact the Dargaville Police, a spokesman said. DoC has gathered statements from five or six people who were in the vicinity around the time of the shooting.
Visser said there had been a lot of talk amongst the west coast community and on social media.
She was confident whoever killed the rare seal would be caught, and she was grateful for how the Dargaville community had made it very clear the act was horrific and unacceptable.
The Sea Shepherd organisation has offered a $5000 reward for information leading to the arrest and prosecution of the shooters.
All seals are protected under the Wildlife Act and Marine Mammals Act, with breaches earning up to two years' imprisonment and/or fines of up to $250,000.
There might be firearms charges or other criminal charges laid in the case of the slaughtered leopard seal.