“It was pure panic,” said Brittany Nicely, 29, who was there with her two children.

Ms. Nicely said that out of the corner of her eye she spotted the boy on the wrong side of wooden and wire barriers along the edge of the enclosure. She reached for him, she said, but not in time, and he fell about 10 feet into a shallow moat.

Harambe, a teenage western lowland gorilla, then grabbed the child, but his intentions were not clear. At times, he stood over the boy in what appeared to be a protective posture. But then he darted across the water, dragging the child roughly behind him.

“He wasn’t throwing him around,” Ms. Nicely said. “He wasn’t mauling him to death, which is what I thought was going to happen.”

The gorilla could have easily crushed the boy at any moment, however, if only by carelessness, she said.