The mother of a three-year-old child who fell into a gorilla enclosure at Cincinnati Zoo will not face charges.

The 17-year-old endangered western lowland silverback gorilla,named Harambe, was shot and killed by staff at the zoo on May 28 after dragging the boy through the water.

Police were investigating the actions of the family in the lead up to the shooting, but Ohio prosecutor Joseph Deters confirmed the mother would not face charges at a press conference on Monday.

The family said they were very pleased with the decision.

The killing of Harambe triggered intense criticism of the zoo and the boy's parents, with over 500,000 people signing a petition asking for 'Justice for Harambe'.

Meanwhile a separate group supporting the mother called 'I support Michelle Gregg', was setup on Facebook.

In a comment published at the time, the boy's mother, Michelle Gregg, said the incident was an "accident" and they "do happen".