(CNN) -- A lion at the Dallas Zoo bit and killed a lioness in their exhibit as visitors watched Sunday afternoon in an attack officials said they couldn't immediately explain.
The 5-year-old lioness, Johari, was bitten in the neck by one of the male lions in her pride around 2:15 p.m., the zoo said.
"At first you think they're playing, and then you realize he's killing her, and ... you're watching it and you just can't believe your eyes," witness Michael Henshaw told WFAA.
"The male lion that started it just had his mouth over her throat, and everyone thought they were playing at first," witness Jim Harvey told WFAA. 'But then they could see she was struggling."
The zoo said Johari died quickly. Witness Dylan Parker told WFAA that the attacking lion held Johari by the neck "for like 10 minutes ... waiting until it quit moving."
The zoo said its keepers closed the exhibit after the incident and took the attacker and three other lions -- his brother and Johari's two sisters -- off the habitat.
Lynn Kramer, a veterinarian and the zoo's vice president of animal operations and welfare, said the attack was a "very rare and unfortunate occurrence."
"In my 35 years as a veterinarian in zoos, I've never seen this happen," he said.
The two surviving lionesses were expected to be put back onto the habitat on Monday, the zoo said. The zoo said the male lions won't be in the exhibit with the lionesses for now, WFAA reported.
Zoo staff said it wasn't clear why the attack happened, according to WFAA.
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