Maris, a 21-year old beluga whale, died suddenly at the Georgia Aquarium on Oct. 22, said zoo officials, announcing her death on Twitter.

"This is a case of sudden, acute animal death. Our animals receive exceptional care, and our dedicated team of experts responded to her within minutes to render aid," said Aquarium Chief Veterinary Officer Dr. Gregory Bossart, speaking on behalf of the Georgia Aquarium, reports USA Today.

Maris had shown no signs of illness, was eating normally, and had interacted well with the Grayson and Qinu, the aquarium's two other belugas, said Bossart in a news release, according to ABC News.

Born in 1994 at the New York Aquarium, Maris arrived at the Georgia Aquarium in 2005.

Meanwhile, rights group PETA, People for the Ethical Treatment of Animals, alleged that captivity was the cause of Maris' death. "Maris was denied her freedom her entire life. She was transferred from one facility to another, and her babies died, one after the other. Whether or not she had a physical ailment that went unnoticed, she was killed by captivity, plain and simple," PETA Senior Vice President Lisa Lange said in a statement Friday, reports CNN.

Maris had a calf that had died soon after birth earlier this year.

An autopsy is underway to determine the cause of death. The results will be out in a few weeks.

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