BROOKFIELD, Ill. (CNN) — Brookfield Zoo suffers another heartbreaking loss.
A 14-year-old lioness, Isis, died on Tuesday.
The lioness was found late Monday lying on the floor, apparently falling and injuring herself.
Zoo officials aren't sure what caused the fall, but was euthanized despite treatment.
The lioness' death comes less than two weeks after her long-time mate, a male lion named Zenda, was euthanized.
According to a statement from the zoo, it was necessary due to "age-related issues that impacted his quality of life."
The two lions had been together for roughly 12 years and often groomed each other, according to the zoo.