DALLAS (CBSDFW.COM) – Sad news out of the Dallas Zoo.

Weeks after welcoming back endangered African painted dogs for the first time in 57 years, the zoo announced 8-year-old Ola was killed overnight by her pack mates.

The zoo explained on Twitter, “Zoologists found her unresponsive this morning behind the scenes. Upon reviewing the closed circuit video feed from the area, we know that she quickly succumbed to injuries sustained during a short bout of aggressive behavior from the two male dogs.”

The hardest part of zoo life and caring for animals is having to share the difficult news of a death. We are saddened to share that 8-year-old African painted dog Ola was killed overnight by her pack mates, 2-year-old brothers Jata and Mzingo. pic.twitter.com/ieVCXfuXhQ — Dallas Zoo (@DallasZoo) July 4, 2019

Introductions had gone very smoothly since the males arrived three weeks ago, and no signs of aggression had been observed between the dogs before this incident, the zoo said.

The zoo went on to explain:

“African painted dogs live in packs with highly complex social dynamics that can shift at any moment, which is unfortunately what happened last night. It’s natural for younger males to test the dominant dog in the pack and to try to establish their own dominance. The boys did nothing wrong here, and were only exhibiting natural behavior. Mzingo and Jata did not sustain injuries and will continue to be in the habitat regularly. Our plan to build out the pack will move forward, and we will work with the SSP on recommendations and timing. We all loved getting to know Ola, and she will be deeply missed. Our animal care team is understandably shaken after this incident, so please send positive thoughts their way.”

When the zoo announced the arrival of the animals they said, “African painted dogs are one of the most social mammals on earth, with the most structured, organized hierarchy of any carnivorous species.”

It’s been a difficult four weeks for the Dallas Zoo. On June 17, Witten the giraffe died while under anesthesia for a physical exam.

On June 4 the zoo announced its oldest male chimpanzee, Doyle, passed away due to age-related health issues.