An African elephant brought to Omaha's Henry Doorly Zoo last year as part of a controversial relocation has died.

Warren, a male African elephant, died Thursday morning after being anesthetized to treat a cracked tusk.The zoo says in a news release Warren is estimated to be between eight or nine years old. Warren arrived at the zoo on March 11, 2016 from Swaziland. He lived as part of a herd at the African Grasslands exhibit.

Warren's cracked tusk worried staff veterinarians because further damage might lead to a possible infection at the root – something that can be fatal in the wild. Natural behaviors such as moving trees and stripping bark are typically the causes of cracked tusks. In July Warren was immobilized to have his tusks trimmed in hopes of resolving the issue. In August new cracks began forming.

The plan was to make a protective metal cap to place on the end of the tusk to prevent additional damage. Approximately 20 minutes into the actual procedure, Warren’s respiration slowed then stopped. Emergency procedures were carried out, but were unsuccessful.

A necropsy will be performed by Omaha’s Henry Doorly Zoo and Aquarium’s Board Certified Veterinary Anatomic Pathologist, Dr. Alison Righton to better understand what may have led to his death. The exact cause of Warren's death is unknown.

Warren was one of 17 elephants flown from Swaziland to the U.S. in early 2016 and placed in the Omaha zoo and those in Wichita, Kansas, and Dallas. Animal rights group Friends of Animals, which opposes elephants being kept in zoos, unsuccessfully sued to stop the importation of the elephants.