Snow leopard shot after escaping from the zoo fence.
Eighteen-year-old animals were euthanized at 17 o'clock on October 23 after Dudley Zoo in West Midland had "another option in the public interest".
The zoo said the snow leopard called Margaash was shot by a senior member of the firearms team and did not suffer.
Margaash, who lived with the three-year-old snow leopard named Taiga, managed to escape after the "guardian's mistake" meant that the fence door remained open.
The Zoo said the incident led to disciplinary actions for those involved.
The death of snow leopard, born at Banham Zoo in Norfolk and transferred to Dudley Zoo in September 2011, prompted Dudley Zoo to conduct an overview of all security solutions.
Visitors at the place on the day of Margaash were euthanized were forced to leave, and those who adopted the animal were notified of his death immediately after the incident.
Director of the Zoological Gardens Derek Grove said, "This was an incredibly sad event, and our staff is understandable to the heart.
"Euthanasia is, and will always be, the last resort. The effort to persuade Margaash to return to his enclosure failed and as the animal was near the surrounding forests and the darkness was approaching, the vet did not believe the ammunition was a safe option until the amount of time the drug is running for.
"Safety of the public is always of paramount importance, and our staff is very experienced and rigorously trained."