The director of the Cincinnati Zoo in the United States says he stands by the decision to shoot a gorilla after a child fell into its enclosure.
Critics are calling for the parents to be charged with child negligence - and the death of an endangered animal.
But the zoo's director Thayne Maynard says the gorilla, named Harambe, had been agitated and disoriented.
He says the decision to shoot saved the boy's life.
"Obviously the child was upset and people were screaming, our security team emptied the exhibit, and the dang animal response team officer came and dispatched Harambe.
"And the reason was that it was a life-threatening situation and a silverback gorilla is a very dangerous animal - it would be very dangerous to us, but particularly a dangerous animal to a little kid.
A petition signed by more than 150,000 people says the boy should have been under proper parental supervision and the mother should be prosecuted.
He said the young boy had crawled through a barrier and fell into the moat.
A YouTube video showed Harambe grabbing the child and shaking him, which was when the security team made the decision to shoot.
Harambe was 17-years-old and weighed 181 kg.
The zoo said tranquilizers were not used because they could have made the gorilla react violently and would have taken time to take effect.
He was born at the Gladys Porter Zoo in Brownsville, Texas, and was moved to the Cincinnati Zoo in 2014.
Western lowland gorillas are classified as an endangered species, and Mr Maynard said the zoo had hoped to use Harambe for breeding.