OMAHA, Neb. (AP) — Henry Doorly Zoo and Aquarium officials say a necropsy performed on a young African elephant that died at the Omaha zoo was unable to determine what caused his untimely death.

The Omaha World-Herald reports that the results of the necropsy came back this week, two months after the elephant — named Warren — died under anesthesia to treat a cracked tusk.

Warren was estimated to be 8 or 9 years old when he died. He 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 in a controversial effort.

Animal rights group Friends of Animals, which opposes elephants being kept in zoos, unsuccessfully sued to stop the importation of the elephants.

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Information from: Omaha World-Herald, http://www.omaha.com