An animal rights group expressed outrage Sunday at the killing of a 400-pound, 17-year-old gorilla to protect a small child that had fallen into a shallow moat at the Cincinnati Zoo.

Two witnesses said Sunday that they thought the ape, named Harambe, was trying to protect the 4-year-old boy before getting spooked by the screams of onlookers. The animal then picked the child up out of the moat and dragged him to another spot inside the exhibit, zoo officials said. 

Fearing for the boy's life, the zoo's dangerous-animal response team shot and killed. 

"Yet again, captivity has taken an animal's life," Julia Gallucci, a primatologist with People for the Ethical Treatment of Animals, said in a statement. "The gorilla enclosure should have been surrounded by a secondary barrier between the humans and the animals to prevent exactly this type of incident.

"Gorillas have shown that they can be protective of smaller living beings and react the same way any human would to a child in danger," Gallucci added. 

Witness Brittany Nicely told ABC News that the gorilla "rushed the boy, but did not hit the boy ... He almost was guarding the boy, was protecting him." Witness Tangie Hollifield agreed, tellingWCPO-TV that Harambe "just held onto him, and went up the ladder and just threw him ... But I don’t think he was hurting him."

Zoo Director Thane Maynard agreed that the gorilla didn't appear to be attacking the child but was "an extremely strong" animal in an agitated situation. He said tranquilizing the gorilla wouldn't have knocked it out immediately, leaving the boy in danger.

"They made a tough choice and they made the right choice because they saved that little boy's life," Maynard said.

The boy's family released a statement Sunday that the boy was home and doing fine.

"We extend our heartfelt thanks for the quick action by the Cincinnati Zoo staff. We know that this was a very difficult decision for them, and that they are grieving the loss of their gorilla," the family said.

Lt. Steve Saunders, a Cincinnati police spokesman, said there are no plans to charge the parents.

Witness Kim O'Connor told WLWT-TV she heard the boy say he wanted to get in the water with the gorillas. She said the boy's mother was with several other young children.

"The mother's like, 'No, you're not. No, you're not,'" O'Connor said.

O'Connor shared video she and her family recorded of the boy and Harambe. The two appear in a corner of the exhibit while visitors yell, "Somebody call the zoo!" and "Mommy's right here!" The station did not air portions of the video showing the gorilla dragging the boy.

Maynard called the killing a tragic death of a critically endangered species and a huge loss for the zoo and the gorilla population worldwide. The gorilla came to Cincinnati in 2015 from the Gladys Porter Zoo in Brownsville, Texas.

Visitors left flowers at a gorilla statue Sunday. Gorilla World remained closed, but the rest of the zoo was open.

The Associated Press contributed to this report.