Zoo chiefs say they are at a loss to explain why a lioness was killed by a lion in front of horrified families.
Five-year-old Johari was bitten on the neck by one of the lions at Dallas Zoo on Sunday afternoon.
Witnesses saw two lions approach Johari at the Texas zoo.
"The male lion that started it just had his mouth over her throat, and everyone thought they were playing at first," Michael Henshaw told Dallas television station WFAA. "But then they could see she was struggling."
Zoo staff were seen throwing meat at the lions to try to distract them, and eventually security moved people away and closed off a restaurant with windows overlooking the lions exhibit.
Lynn Kramer, the zoo's vice president of animal operations and welfare, said a total of five lions were typically in the enclosure and never appeared to endanger each other before.
The three lions involved were in the same enclosure for three years, he said.
"I would have to think something caused the males to react that they don't normally see every day," Mr Kramer told The Dallas Morning News. "Lions can be aggressive, but they don't kill each other."
The zoo says the aggressive lions were not put down.