AKRON, Ohio (WJW) — Akron Zoo representatives said they were “heartbroken” by the unexpected deaths of the zoo’s young snowy owls, which hatched just weeks ago.

The owlets died in the early morning of Tuesday, Aug. 23, according to a news release from the zoo.

Autopsy results to determine their cause of death were inconclusive. Further tests are expected. The owlets had no visible signs of injury or illness. Snowy owls have a 33% mortality rate in their first year of life, according to the zoo.

Their hatchings on July 31 and Aug. 4 were “a first” for the zoo, according to the release.

The zoo’s adult snowy owls, the male Frost and the female Cirrus, remain in good health and can be seen every day in the zoo’s snowy owl habitat.

“We are heartbroken at the sudden passing of our two owlets,” Shane Good, the zoo’s senior director of animal care, is quoted in the release. “Our avian care team worked tirelessly to provide a comfortable environment for our snowy owls to breed, and we will continue to offer Frost and Cirrus the best care. The unexpected loss of our owlets is very deeply felt by our staff, who dedicate their lives to providing exceptional care to all our animal residents every day.”

Snowy owls are considered to be vulnerable to extinction by the International Union for Conservation of Nature and Natural Resources. The Akron Zoo participates in a breeding plan to ensure the species’ genetic diversity across more than 200 accredited zoos like Akron’s, according to the release.