On Sunday, a male lion attacked and killed a five-year-old lioness named Johari in front of dozens of stunned families at the Dallas Zoo. Zookeepers are still unsure of what caused the shocking incident.

"At first you think they're playing; then you realize he's killing her... and you're watching it," Michael Henshaw, a spectator, told WFAA. "You just can't believe your eyes."

"The male lion that started it, just had his mouth over her throat, and everyone thought they were playing at first but then they could see she was struggling," Jim Harvey, another spectator, told WFAA.

"[The lion] just lay beside her and held her by the neck for like 10 minutes... just holding it there, waiting until it quit moving," Dylan Parker, a third spectator, told the CNN affiliate.

Zookeepers soon closed their Giants of the Savanna exhibit. Johari was part of the exhibit since it opened in 2010 and had lived with the two male lions for three years, CBSDFW reports. A necropsy determined that Johari died of suffocation.

"There was a couple of puncture wounds, and that was all we saw externally...and there was some hemorrhaging underneath the skin," Lynn Kramer, a veterinarian and the zoo's vice president of animal operations and welfare, told CBSDFW. "Basically the animal's trachea was collapsed and she suffocated."

According to Kramer this circle of life display is "very rare and [an] unfortunate occurrence."

"In my 35 years as a veterinarian in zoos, I've never seen this happen," Kramer said.

According to WFAA, Dallas Zoo's remaining two lionesses will go back to their exhibit today. Male lions will not be placed with the lionesses, at least for now.

The male lion was neither euthanized nor tranquilized, but instead was moved from the enclosure.

Officials will investigate the incident by trying to figure out what Johari's behavior was like before she was killed. The investigation could take months.

"We deeply appreciate the outpouring of support we're receiving after the loss of our beloved Johari. Rest assured, your messages will be shared with our entire staff, and will help them greatly during this time of grief," Dallas Zoo said via Facebook.

Check out the CNN's coverage of the incident which features spectator footage of the attack on Johari:

Related