The mother of a toddler who tumbled into a gorilla exhibit will not be charged with child endangerment. "If anyone doesn't believe a 3-year-old can scamper off very quickly, they've never had kids, because they can and they do," said Hamilton County Prosecutor Joseph Deters in a Wall Street Journal report.

The Ohio child endangerment statute requires that a parent recklessly create a substantial risk to the health or safety of a child.

"If the mother had been in a bathroom smoking crack and running around the zoo, that would have been a different story. She was being attentive to her children by all witness accounts," Deters added.

The boy's relatives expressed their relief, saying: "The family is very pleased... it is what we expected.

"This is one more step in allowing us to put this tragic episode behind us and return to our normal family life."

The accident happened on 28 May, when the boy fell over 12 feet into the moat at the Gorilla World exhibit and was dragged around by Harambe, a 17-year-old silverback. The large ape was shot dead by zoo staff, causing an outpouring of public condemnation, questioning whether the mother should be held accountable.

An online petition criticising Michelle Gregg, the boy's mother, and received around 500,000 signatures, with a Facebook group called Justice for Harambe set up.

Deters defended the zoo's decision to shoot the animal at a news conference: "The one thing I will say, the zoo lost a beautiful animal, one that many people in this area have enjoyed watching for a long time, but it's still an animal.

"It does not equate human life," he added. "And they felt that this boy's life was in jeopardy, and they made the painful choice to do what they did."

Cincinnati Zoo is set to reopen the Gorilla World exhibit on Tuesday 6 June, constructing a higher barrier in further safety measures to prevent such accidents happening again.

"Our exhibit goes above and beyond standard safety requirements, but in light of what happened, we have modified the outer public barrier to make entry even more difficult," said Thane Maynard, director of the Cincinnati Zoo.