The male seized five-year-old Johari by the neck on Sunday at the Dallas Zoo - an autopsy（验尸） found she died from neck wounds and haemorrhaging（出血）.
Zoo officials said the lions had lived together peacefully for years.
They said they had no idea why the male had turned on Johari, but that they have no plans to euthanize（使安乐死） him.
Video taken by a zoo visitor showed the apparently calm male lion with its jaws clamped around the female's neck.
"Everyone thought they were playing at first but then they could see that she was struggling," witness Jim Harvey told local news station WFAA.
'Very rare occurrence'
Security personnel closed the exhibit to the public and the male was removed.
The remaining four lions appeared unaffected by the incident, said officials.
"Johari was a remarkable animal, as are all of our lions" said Lynn Kramer, vice-president of animal operations and welfare at the zoo, in a statement.
"This is a very rare and unfortunate occurrence. In my 35 years as a veterinarian（兽医） in zoos, I've never seen this happen."
The statement said Johari, known as Jo-Jo, was a staff favourite who was "sweet and loving with her sisters, and often could be found grooming them".
Mr Kramer told WFAA that male lions do sometimes kill other males in the wild, and sometimes cubs, but that attacking a female was extremely rare.
The chances of such an attack happening again were "very remote", said Mr Kramer.
However, the zoo has said it will keep its two males lion apart from the surviving two female while they try to establish what happened, and would "absolutely not" euthanize the killer.
.gif)
Something wrong with that lion