The boy's head was banging on the concrete as he was being dragged through the enclosure by the 204kg animal, which was one factor in the decision to shoot Harambe, the Western lowland gorilla, whose species is listed as endangered.
"Looking back, we would make the same decision" to shoot the gorilla, Thayne Maynard, director of the Cincinnati Zoo & Botanical Gardens, told a news conference on Monday.
"The gorilla was clearly agitated. The gorilla was clearly disoriented," he said.
Outrage is mounting over the killing. More than 200,000 people had signed online petitions on Change.org to protest the shooting. Some petitions urged police to hold the child's parents accountable.
The most popular "Justice for Harambe" petition seeks police action and also urges child protective services to investigate the boy's home to guard against "further incidents of negligence."
Witnesses said they heard the boy telling his mother he wanted to enter the enclosure. The child, whose name has not been released, then slipped behind a barrier and fell about 4.5 metres into the habitat.
People laid flowers and commemorative notes at a gorilla statue inside the zoo, and local media reported that activists planned a vigil on Monday.
Michelle Gregg, who identified herself on Facebook as the boy's mother, said her son was recovering from a concussion and a few scrapes. She asked others not to judge her because "accidents happen."
A family statement on Sunday expressed condolences to the zoo for the loss of 17-year-old Harambe.
"We are so thankful to the Lord that our child is safe. He is home and doing just fine," the statement said.
"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 zoo's dangerous animal response team shot Harambe dead about 10 minutes after he encountered the child.
It was the first time in the 38-year history of the Cincinnati Zoo & Botanical Garden's gorilla exhibit that an unauthorised person entered the enclosure, Maynard said.
"We are heartbroken about losing Harambe, but a child's life was in danger and a quick decision had to be made.
"Tranquilisers were not an option because they could have a delayed effect and "the impact from the dart could agitate the animal and cause the situation to get much worse."
Maynard maintained that the exhibit was safe and the zoo was not negligent. The boy apparently climbed over a 1-metre barrier, he said.
"People can climb over barriers. That's what happened. The barrier's adequate."
However, the zoo would review the barrier to see if it could be improved, he said.
