In just two weeks, the Brookfield Zoo in Illinois has been forced to euthanize both of its African lions.

On Tuesday, the Chicago Zoological Society announced that Isis, a 14-year-old female lion, was found injured and lying on the floor of her enclosure's moat. Officials believe Isis fell sometime Monday morning after animal care staff checked on her around 9 a.m.

Despite intensive treatment by the veterinary staff, she reportedly suffered "significant injuries" and "the difficult decision was made" to humanely euthanize her, according to the zoological society.

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“The safety and well-being of the animals in our care is our utmost priority,” said Bill Zeigler, senior vice president of animal programs. “Although it is unclear what caused her fall, we are reviewing all aspects of this tragic occurrence.”

Isis's passing was devastating for zoo staff considering her longtime mate Zenda -- Brookfield's 13-year-old male African lion -- was euthanized Jan. 2 after his health had declined dramatically.

He would have turned 14 in July.

"Just before Christmas, animal care staff observed Zenda having difficulty standing and walking," a Jan. 3 news release said. "Over the past week, veterinarians had been treating his symptoms with medication to help ease the pain. Preliminary necropsy results revealed Zenda had several degenerative discs in his spine that had ruptured as a result of his advanced age and were inoperable."

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Both lions had arrived at the zoo in May 2008 and were routinely seen "grooming one another other and sleeping together," according to Tuesday's release.

They were also reportedly loved "by staff and visitors alike."

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“Isis and her mate were majestic animals who had a strong bond, said Zeigler. "The staff, who dedicated their lives to care for these charismatic lions, are heartbroken by their loss."