MINNEAPOLIS (WCCO) — Minnesota Zoo officials say one of their popular Amur tigers, Nadya, unexpectedly passed away over the weekend.

According to zoo officials, Nadya hadn’t been feeling well for a week prior to her death. Her medical situation didn’t improve, so an emergency exploratory surgery was initiated to determine her illness.

Nadya survived the surgery, but died soon afterwards, officials said. Blood tests showed that she was suffering issues with her liver, but the full necropsy results are still to be determined.

She was 3 years old.

“We are very sad over the unexpected passing of Nadya,” Kevin Willis, the Minnesota Zoo’s vice president for biological programs, said. “Because she was hand raised here, she was one of our favorite animals and despite receiving the very best in medical care, she unfortunately didn’t survive. She will always be remembered by staff and volunteers as a sweet-natured tiger.”

The Minnesota Zoo currently has four other Amur tigers ranging from 3 to 16 years old. The eldest, Molniy, is currently being treated for arthritis and other age-related conditions. The breed typically lives somewhere between 10 to 15 years.