The 180kg gorilla named Harambe was shot dead by Cincinnati Zoo officials just one day after his 17th birthday.
The shooting has sparked an outcry of emotion as mourners called it a "senseless death".
Many are placing the blame squarely on the parents of a four-year-old boy, who investigators believe crawled through a railing barrier and fell into the gorilla exhibit's moat before he was dragged by Harambe in the water for about 10 minutes.
The small child said he wanted to get in the water before the incident, to which the mother, who was also watching several other children, replied: "No, you're not, no, you're not," according to one witness Kim O'Connor.
The zoo's animal response team assessed the 'life-threatening situation' and defended their decision to shoot Harambe rather than tranquilize him, but thousands took to social media to call it a 'murder'.
O'connor told WLWT she heard the boy talking about getting into the water before she heard a splash, followed by frantic yelling once onlookers realised he was inside the enclosure.
A video emerged on Saturday revealing some of the chilling moments Harambe was dragging the boy in the water, although more graphic portions were cut from the footage.
The panicked bystanders may have aggravated the tense situation, according to O'Connor, who said: "I don't know if the screaming did it or too many people hanging on the edge, if he thought we were coming in, but then he pulled the boy down away further from the big group."
In the video that emerged on Saturday, a woman can be heard yelling, "Mummy's right here...mummy loves you,' and before saying 'Isaiah be calm,' when the boy started crying.
Director Thane Maynard supported the zoo's dangerous animal response team for their decision to put down the gorilla.
"They made a tough choice and they made the right choice because they saved that little boy's life," Maynard said.
But outraged animal lovers took to social media declaring the western lowland gorilla's life was unnecessarily taken, and more than 1,000 have already joined the Facebook group Justice for Harambe.
While some defended the parents, many others were less sympathetic.
One Twitter user wrote: "So a beautiful, innocent gorilla has to die because neglectful parents can't control their kids? Mankind sucks :( #Harambe #CincinnatiZoo"
Another user Chris Dasauchoit tweeted: "Beautiful animals sadly paying for utter human stupidity and negligence with their lives. #Harame #CincinnatiZoo."
Zoo officials said three gorillas were in the enclosure when the boy fell in the moat, but the two female gorillas were called out immediately.
Harambe remained in the yard with the child.
Some said Harambe appeared to be guarding and defending the boy, but video footage also showed him dragging the four-year-old in the water.
According to Maynard, the gorilla did not appear to be attacking the child, but he called it "an extremely strong" animal in an agitated situation.
"You're talking about an animal that's over 400 pounds and extremely strong. So no, the child wasn't under attack but all sorts of things could happen in a situation like that. He certainly was at risk," he told WLWT.
According to a fire department incident report, the gorilla was "violently dragging and throwing the child", who was between Harambe's legs when the gorilla was shot, WLWT reported.
Maynard explained that tranquilising the gorilla would not have knocked it out immediately, leaving the boy in danger.
The child was taken to Cincinnati Children's Hospital Medical Center with serious but "non-life threatening" injuries following the incident, which was reported around 4pm.
He was still alert when he was taken to hospital, according to officials.
Officials said they could not release any information on the child, including his name.
Zoo director Maynard noted it was the first time that the team had killed a zoo animal in such an emergency situation, and he called it "a very sad day" at the zoo.
He said said in a statement: "The Zoo security team's quick response saved the child's life.
"We are all devastated that this tragic accident resulted in the death of a critically-endangered gorilla. This is a huge loss for the Zoo family and the gorilla population worldwide."
Western lowland gorillas are deemed critically endangered by the World Wildlife Fund.
Harambe came to Cincinnati in 2015 from the Gladys Porter Zoo in Brownsville, Texas. According to the zoo's website, it houses 11 gorillas.
The area around the gorilla exhibit was closed off on Saturday afternoon as zoo visitors reported hearing screaming.
The zoo is to be open as usual today but Gorilla World will be closed until further notice.
In March, two curious polar bears at the zoo wandered into a behind-the-scenes service hallway through an open den door, but never left a secondary containment area.
The zoo said the 17-year-old female Berit and the 26-year-old male Little One, entered an "inappropriate" area but remained contained and were never loose or a threat to the public.
During that incident, zoo officials said staff followed protocols and safely returned the bears to their main holding area within two hours.
