Police have said the parents of a four-year-old who climbed into a gorilla's enclosure could face criminal charges after staff were forced to kill the beloved animal.

Harambe the 400lb gorilla was shot dead by Cincinnati Zoo officials just one day after his 17th birthday when the boy climbed through barriers and fell into the enclosure.

The incident, which was captured on cell phone, has sparked an outcry of emotion, with thousands of mourners branding it a 'senseless death'.

Many are placing the blame squarely on the parents of the boy. They are yet to be charged but police said prosecutors could choose to indict them.

Michelle Gregg, the mother of the boy, posted a message on Facebook saying: 'I want to thank everyone for their thoughts and prayers today. What started off as a wonderful day turned into a scary one.

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A special zoo response team shot and killed a 17-year-old gorilla named Harambe (pictured) that grabbed and dragged a four-year-old boy who fell into its gorilla exhibit moat, the Cincinnati Zoo's director said. His parents are now facing criminal charges

Footage taken by another visitor shows the gorilla grabbing on to the boy's shirt. Many are placing the blame squarely on the parents of a four-year-old boy, who investigators believe crawled through a railing barrier

'For those of you that have seen the news or been on social media that was my son that fell in the gorilla exhibit at the zoo. God protected my child until the authorities were able to get to him.

'My son is safe and was able to walk away with a concussion and a few scrapes... no broken bones or internal injuries.

'As a society we are quick to judge how a parent could take their eyes off of their child and if anyone knows me I keep a tight watch on my kids. Accidents happen but I am thankful that the right people were in the right place today.'

The family released a statement on Sunday saying they had taken their boy home.

It read: 'We are so thankful to the Lord that our child is safe. He is home and doing just 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. We hope that you will respect our privacy at this time.'

A mother who was at the zoo said she tried to stop the child from going into the enclosure but couldn't grab him in time.

Brittany Nicely told WHIO the gorilla was also acting protectively towards the boy and was not behaving in a threatening manner.

'I tried to prevent it, I tried to grab him and I just couldn’t get to him fast enough,' she said.

The zoo shot the beloved animal after he dragged the boy through the water, but many say he was simply trying to protect the child

The zoo's Gorilla World will be closed until further notice. Flowers and commemorative notes were left at a gorilla statue in the zoo on Sunday

'What the first responders saw, I’m just not sure... They said he was violently throwing the child around, which seems crazy to me.

'They have a picture of the boy sitting in front of the gorilla moments before they shot him.'

A day after the incident, she said her kids are traumatized and do not want to go back to the zoo.

Deidre Lykins was also at the zoo when she saw the boy drop into the enclosure.

She described how Ms Gregg was calling out for her son and had just been next to him when he disappeared.

Then she had to stop her husband from going in to try and rescue him. But she insists Ms Gregg is not at fault.

She wrote on Facebook: 'This was an open exhibit! Which means the only thing separating you from the gorillas, is a 15 ish foot drop and a moat and some bushes!

'This mother was not negligent and the zoo did an awesome job handling the situation! Especially since that had never happened before!

'Thankful for the zoo and their attempts and my thoughts and prayers goes out to this boy, his mother and his family.'

More than 2,000 people have already signed a Change.org petition calling for the boy's parents to be 'held accountable for their actions of not supervising their child' - and slamming the zoo for putting Harambe down.

Brittany Nicely (left and right) said she tried to stop the child from going into the enclosure but couldn't grab him in time. She then tried to calm the boy's mother has the chaos unfolded

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

The child said he wanted to go in the water moments before the incident, eyewitness Kim O'Connor said.

Ms Gregg, who was also watching several other children, is said to have replied: 'No, you're not, no, you're not.'

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 when onlookers realized he was inside the enclosure.

Chilling footage showed Harambe picking up the boy and dragging him through the water, but more graphic portions were cut from the video.

According to O'Connor, the gorilla looked like he was trying to protect the boy from panicked bystanders who may have aggravated the tense situation.

She 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, 'Mommy's right here... mommy loves you', before adding, 'Isaiah be calm', when the boy started crying.

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

The zoo celebrated Harambe's birthday on Friday, just one the day before he died (left). His death has sparked an out pour of emotions, with many calling it a 'murder' and 'senseless death' (right)

Jerry Stones, who worked at the Gladys Porter Zoo, in Bronwsville, Texas, where Harambe was raised, said: 'It tore me a new one. An old man can cry, too. He was a special guy in my life. It's a sad day for us'

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 3,000 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.'

Many are placing the blame squarely on the parents of a four-year-old boy

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.

According to Maynard, the gorilla did not appear to be attacking the child, but he described it as '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 tranquilizing 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.

Zoo director Thane Maynard supported the response team's decision to put down the gorilla, but many disagreed. More than 1,000 people have already joined the Facebook group Justice for Harambe

Harambe came to Cincinnati in 2015 from the Gladys Porter Zoo in Brownsville, Texas. He is a western lowland gorilla, which the World Wildlife Fund deemed critically endangered

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 and condition.

Zoo director Maynard noted it was the first time the team had killed a zoo animal in such an emergency situation, and he called it 'a very sad day'.

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.'

Jerry Stones, who worked at the Gladys Porter Zoo, in Bronwsville, Texas, where Harambe lived before he was transferred in 2015, said he was devastated by the news.

Stones, who raised the gorilla, told the NY Daily News: 'It tore me a new one. An old man can cry, too. He was a special guy in my life. It's a sad day for us.'

He added: 'He grew up to be a pretty, beautiful male. He was very intelligent. His mind was going constantly. He was just such a sharp character.'

Western lowland gorillas are deemed critically endangered by the World Wildlife Fund.

The child was taken to Cincinnati Children's Hospital Medical Center with serious injuries following the incident

Primatologist Julia Gallucci issued a statement through animal-rights group PETA, saying: 'Yet again, captivity has taken an animal's life.

'The gorilla enclosure should have been surrounded by a secondary barrier between the humans and the animals to prevent exactly this type of incident.'

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 on Sunday 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.