(CNN)A 14-year-old female African lion of an Illinois zoo died Tuesday -- less than two weeks after the death of her mate, the Chicago Zoological Society said in a statement.
Brookfield zoo staff checked on Isis, the lioness, Monday morning and everything appeared normal, the statement said.
"She was fine, she was moving around," Senior Vice President of Animal Programs Bill Zeigler told CNN affiliate WTTW.
In fact, she hadn't been having any medical or health issues, he told the affiliate.
But when staff checked back later in the day Monday she was laying on the floor of the moat, after apparently falling and injuring herself.
"Despite the immediate and intensive treatment provided by the veterinary staff, Isis sustained significant injuries and the difficult decision was made to humanely euthanize her today," the organization said.
It called Isis' death "sudden."
"Although it is unclear what caused her fall, we are reviewing all aspects of this tragic occurrence," Zeigler said in a statement.
Isis' mate, the zoo's male African lion named Zenda, had been euthanized about two weeks ago, the organization said, because of "age-related issues that impacted his quality of life." He was 13 years old and had been having trouble standing and walking, the zoo said in a tweet earlier this month.
The two had a strong bond, Zeigler said. They had been in the zoo for about 12 years and were often grooming each other and sleeping together.
"The staff, who dedicated their lives to care for these charismatic lions, are heartbroken by their loss," Zeigler said.