The incident, which involved two male lions and a lioness, happened around 2:15 p.m. Sunday, WFAA-TV, Dallas/Fort Worth, reported.
"At first you think they're playing; then you realize he's killing her ... and you're watching it," said Michael Henshaw, who was visiting the zoo Sunday and saw the incident. "You just can't believe your eyes."
Jim Harvey, another zoo guest, said the lions seemed to be playing at first.
"The male lion that started it just had his mouth over her throat, and everyone thought they were playing at first," Harvey recalled. "But then they could see she was struggling."
Lynn Kramer, the vice president of animal operations and welfare at the zoo, said all three lions were born in captivity and had been living together at the zoo for the past three years.
"We've had, you know a few incidents of rough play, but nothing out of the ordinary," Kramer said.
Kramer added that it was a rare event, as lions, including those living in the wild, rarely turn on each other.
"I've been in the zoo business over 35 years," Kramer said. "I've worked in five major zoos, and I've never seen a cat kill another cat before."
The zoo said the male lions involved were not euthanized or tranquilized after the attack, but were moved out of the viewing area. For now, no other lionesses will be placed in the exhibit, the zoo said.