Fresno County Coroner David Hadden said Dianna Hanson, 24, was talking on her phone while cleaning the pen at Project Survival's Cat Haven in Dunlap Wednesday afternoon when a 5-year-old male lion named Cous Cous lifted up the cage door with his paw, sneaked up behind her and broke her neck, the San Francisco Chronicle reported.
She died almost immediately, but the big cat then played with her body "much the way a cat would play with a mouse," the coroner said.
"There were other scratch and bite injuries, which were all postmortem," he said. "The young lady did not suffer."
Hadden said sheriff's investigators told him it appeared the cage door wasn't properly secured. The Chronicle said investigators said they would not comment until they had completed their investigation.
Hadden said the co-worker came to investigate after their phone conversation ended abruptly and found the lion standing over Hanson's body.
Sheriff's deputies then shot the lion to death because he would not give up Hanson's body. The lion had been at the refuge since it was 8 months old.
The sanctuary was closed to the public Wednesday and founder Dale Anderson said he was not there when the attack occurred. He said "safety protocols were and are in place."
Anderson said the deadly incident was "devastating."
"You start realizing I lost two really good friends and it is hard," he told the Chronicle. "I can't a put circle around it. I can't tie it off."
Sanctuary President Wendy Zebbas said Hanson, a Western Washington University graduate who majored in biology, "loved big cats."
"She was doing what she loved and she did it with joy every day she worked here," Zebbas said.
Hanson had three years of experience working with big cats at a private tiger reserve in Bellingham, Wash., her father said.
The Human Society said Hanson should never have been allowed in big cat's enclosure, The Fresno Bee reported.
Nicole Paquette, vice president of wildlife for the Humane Society of the United States, said lions can be dangerous, describing them as "ticking time bombs waiting to explode."
"It's completely irresponsible to allow someone to go into the enclosure with a dangerous wild animal," she said of the incident.
Lt. Tony Spada, a local state Fish and Wildlife warden, said his department and the sheriff's office would investigate the attack.
"This place has a good history -- up to this point," Spada said. "This is a situation where somebody was too close to a lion."