CHICAGO (CBS) – The dolphin born at the Shedd Aquarium last week has died, directors announced Friday.

The female Pacific white-sided dolphin calf was born June 3 to the Shedd’s 26-year-old dolphin mother Tique, which had been transported to a Miami dolphin habitat to be impregnated. The young dolphin died Thursday.

Even after a necropsy, Shedd officials said they aren’t sure how the animal died. They noted the calf had taken longer than expected to learn how to nurse from its mother.

“This is an extremely sad day for the Shedd Aquarium family,” Ted A. Beattie, Shedd’s president & CEO, said in a news release.  “Over the last week, our expert marine mammal and veterinary staffs provided around-the-clock care for the calf. From the moment the calf was born, they worked tirelessly to provide Tique and the calf with the best possible care. It has been a difficult loss for us.”

A cause of death may never fully be known, Shedd officials said.

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