Nadya, a 3-year-old Amur tiger at the Minnesota Zoo, died unexpectedly over the weekend, zoo officials said.

The tiger had been ailing the week before her death, but veterinarians and zookeepers could not determine what caused her illness.

As her situation worsened, she underwent emergency exploratory surgery, which was inconclusive. She died after surgery.

The zoo is awaiting a necropsy, or animal autopsy, from the University of Minnesota, to solve the mystery of her death.

"We are very sad over the unexpected passing of Nadya," Kevin Willis, the Minnesota Zoo's vice president for biological programs, said in a news release. "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."

When Nadya became lethargic and stopped eating, a blood test revealed compromised liver function, Willis said. She was given pain medication, but still would not eat. On Saturday, the zoo conducted a laparoscopy on Nadya and then surgery with the help of BluePearl's specialty and emergency veterinarians.

When the anesthetic was withdrawn, Nadya went into cardiac arrest. Officials resuscitated her and shifted her back into the holding building, where she went into cardiac arrest again.

Nayda had no preexisting conditions, Willis said. "This one stands out to me because she was so young," he said.

Nadya came to the Minnesota Zoo in 2012 as a cub. She arrived from the St. Louis Zoo to be with Sundari, a cub born in Minnesota. The mothers of both cubs could not nurse them, so staff from the zoos intervened to hand-raise the cubs. It was decided it would be best if the cubs were brought up together at the Minnesota Zoo. So Nadya and Sundari, or "Dari," lived in their exhibit together in the zoo.

"They were raised as siblings,' Willis said. "It isn't really normal for adult tigers to be together, but since they were together as cubs and raised together. They were just used to each other and got along."

Unlike the other tigers, Nadya could be friendly, Willis said. When she saw her keepers, she would chuff, a sound similar to purring.

"There are some tigers, when you walk into the barn, they crouch, they hiss, sometimes they roar," Willis said. " She would greet you."

On social media Monday, zoo members shared memories of Nadya and offered their condolences.