ASHEBORO, N.C. -- Veterinarians at the NC Zoo had to euthanize the 21-year-old gorilla named "Nkosi" Tuesday.
He started showing symptoms of an unidentified illness a week ago. He underwent treatment, but the veterinarians couldn't get him back to good health.
They are doing a necropsy - an animal autopsy - to find out what caused the illness.
Nkosi, called Nik by trainers at the zoo, was best known for fathering two baby gorillas in 2012. They were the first gorilla births at the zoo in 23 years.
The other gorillas are still in the exhibit.