No charges will be brought against the mother of a boy who gained access to a Cincinnati Zoo enclosure, prompting the shooting of an endangered gorilla, a US prosecutor has said. 

Harambe, a 17-year-old male gorilla, was killed on May 28th after grabbing the three-year-old boy. 

The child, who had been visiting the zoo with his mother and relatives, was dragged around the enclosure for a number of minutes before staff shot the gorilla.  

Over half a million people have since signed an online petition calling for a police investigation into the family's "negligence".

But local county prosecutor Joseph T Deters confirmed in a statement today that the boy's parent would not be prosecuted. 

"Many people on social media have condemned the mother for not watching her child carefully enough at the zoo," he said.  

"None of the witnesses interviewed [by police] described the mother as anything but attentive to her children. 

"Our information is that the mother turned away for a few seconds to attend to another one of her young children and that is when the three-year-old was able to climb into the gorilla enclosure. 

"Any parent who is honest with himself or herself would have to understand how this could happen to even the most attentive parent."

Mr Deters added: "I am very sorry about the loss of this gorilla but nothing about this situation rises to the level of a criminal charge.

“This was a tragic accident and a terrible loss for the zoo of their beloved gorilla. 

"However, the zoo did the right thing when they took immediate action to save the life of a young child.  A gorilla is a wild animal and, by definition, dangerous and unpredictable."