Horrified visitors at Dallas Zoo looked on as two male lions attacked and killed a lioness in their enclosure this weekend.
The incident occurred at the Savannah Exhibit area of the park.
Two male lions, named Denari and Kamaia, had lived with the female, Johari, for three years.
Officials said there were no warning signs that Sunday's attack might occur.
Vide President of animal operations at Dallas Zoo, Dr Lynn Kramer, told CBC Local: "We've had a few incidents of rough play, but nothing out of the ordinary.
"We really don't know what caused this. There were no external wounds on this cat, so we think he just grabbed her by the throat and it was very quick."
Dr Kramer said in his 35 years of working in the zoo business, this is a first.
"I've worked in five major zoos, and I've never seen a cat kill another cat before," he said.
One witness, Dylan Parker, 19, said he saw the largest of the lions corner Johari.
He told Dallas News the animals started fighting, and "the smaller male lion pinned the female down and held it so it quit moving".
He said there was one zoo keeper outside the enclosure who initially thought they were playing. He added: "Then they started yelling at the lion, and [we] saw them throw chunks of meat to try and distract them".
In a written statement, the zoo said they were "heartbroken" at the loss of Johari, who was described as a "playful and loving" cat.
The lions have been taken away from the public exhibit, and the zoo says they will be working with the Lion Species Survival Plan to determine what, if anything, will happen to the two male lions involved.