Authorities believe a lion broke the neck of a young volunteer at a US animal park after opening its cage with its paw and attacking as the woman cleaned its holding area.
But they do not yet know how the lion and 24-year-old Dianna Hanson came to be in the same place at the same time, Fresno County coroner David Hadden said.
"The lion had been fed, the young woman was cleaning the large enclosure, and the lion was in the small cage. The gate of the cage was partially open, which allowed the lion called Cous Cous to lift it up with his paw," Mr Hadden said, based on a briefing from investigators. "He ran at the young lady."
The lion broke Ms Hanson's neck, probably with a swipe of a paw, killing her instantly, he said.
Ms Hanson was talking with a co-worker on a mobile phone in the moments before she was killed on Wednesday, the coroner said. The co-worker became concerned when the conversation ended abruptly and Ms Hanson failed to call back. The co-worker then called authorities when she went to check on Ms Hanson.
Sheriff's deputies shot Cous Cous after he could not be coaxed away from Ms Hanson's body.
The intern sustained numerous bites and scratches and a post-mortem examination revealed they were inflicted after she died. "Which means the young lady ... wasn't alive when the lion was tossing the body about," said Mr Hadden.
Ms Hanson had been working for two months at Cat Haven, a private zoo in California. Her father, Paul Hanson, described her as a "fearless" lover of big cats. She died doing what she loves, he said. She told her father she was frustrated that Cat Haven did not allow direct contact with animals. "She was disappointed because she said they wouldn't let her into the cages with the lion and tiger there," Mr Hanson said.
The owner of the zoo said safety protocols were in place, but he would not discuss them because they are a part of the law enforcement investigation. Dale Anderson said he is the only person allowed in the enclosure when lions are present. "We have been incident-free since 1998 when we opened," Mr Anderson said.
The US Department of Agriculture, which enforces the federal Animal Welfare Act, is also looking to understand why the lion turned on the intern. USDA inspectors conduct multiple unannounced inspections of Cat Haven every year and had never found a violation, spokesman Dave Sacks said.