LANSING, MI - Potter Park Zoo is mourning the loss of its 5-year-old Alaskan moose, Willow, which it had raised since it was a youngster, after an unknown illness left the animal unable to get up this week.

Zoo caretakers found Willow unable to stand on Friday morning. Veterinary staff tried to treat her, but determined nothing could be done. They described the decision to euthanize Willow to keep her from unnecessary suffering as "excruciating," according to the Associated Press.

While zoo staff say they don't know what caused Willow's sudden decline, a necropsy is being done at Michigan State University and results are expected in a few weeks.

"The zoo team is deeply saddened by this tragic and unexpected loss," said Zoo Director Cynthia Wagner in a message posted on the zoo's blog.

"Willow was an incredible ambassador for her species and was loved by all who visited Potter Park Zoo. She will be deeply missed by our team."

Willow had been with the zoo since 2013. She was born in the wild, along with the zoo's male Alaskan moose, Meeko.

The two were rescued after being separated from their mothers. Willow was brought to Potter Park first, and Meeko arrived in 2014.

A special exhibit for the moose was unveiled in 2016.