A lion killed a lioness over the weekend at Dallas Zoo in Texas, and zookeepers aren’t quite sure why. Dozens of families watched in horror as the male lion grabbed the female lioness by the throat and held her down until she stopped moving. The deadly incident happened around 2:15 p.m., according to CNN.

The lioness, named Johari, was 5 years old when she was killed on Sunday. She lived in the Savannah Exhibit of the park and shared a space with four other lions -- two other females and two males. The males, Denari and Kamaia, were living with Johari for three years. According to a representative of the zoo who spoke with CBS, the lions were involved in “rough play” in the past, but were not known for violent attacks like the one that occurred over the weekend.

"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," Michael Henshaw, who witnessed the lion kill the lioness, told WFAA.

Spectators described how the lion held the lioness down for 10 to 15 minutes until she stopped moving. Many witnesses said they didn’t know what was happening at first, until the female lion went limp. The other male lion also joined in on the attack, according to The Huffington Post. Zoo officials are still puzzled over what exactly provoked the attack.

"This is a very rare and unfortunate occurrence,” Lynn Kramer, vice president of animal operations and welfare at the Dallas Zoo, told reporters, according to Reuters. “In my 35 years as a veterinarian in zoos, I've never seen this happen.”

Lions are the only social felines. While aggressive animals, they rarely attack animals of their own pride. Then again, “The central insight of [Craig] Packer’s career is this: Lions evolved to dominate the savanna, not to share it,” Smithsonian Magazine reported in a story dealing with Packer, a Minnesota ecologist and lion expert.

“I would have to think something caused the males to react that they don’t normally see every day,” Kramer told Dallas News. “Lions can be aggressive, but they don’t kill each other.”

The Dallas Zoo tweeted about the incident after it happened, saying zoo staff were “heartbroken” over the death of the lioness. “A female lion has died today after a fight with another lion,” the zoo tweeted. “Please send good wishes to our staff.”

The zoo also posted a photo of the lioness to Twitter, and thanked their followers for their words of support. The zoo also tweeted that there was “no indication…from her exams and the necropsy” that the female lion was sick, after one Twitter user suggested she may have been ill. “We probably will never know the exact catalyst,” the zoo responded.

Some people speculated whether or not the Dallas Zoo would euthanize the lion who killed the lioness. But the zoo said they were not planning on it. “We won't euthanize animals for being true to their wild nature,” a zoo spokesperson tweeted.