Hamilton County Prosecutor Joe Deters said the child's mother had three other children with her, and she was attending to them when the 3-year-old "just scampered off" on May 28. He said children's services made a visit to the boy and mother, and social workers were impressed by the child's environment.
Deters said the mother's actions were "not even close" to meriting reckless endangerment charges.
The zoo plans to reopen its Gorilla World on Tuesday with a higher, reinforced barrier. The boy apparently climbed over the outer barrier before falling some 15 feet into a shallow moat.
A special response team shot and killed the 17-year-old western lowland gorilla named Harambe to protect the boy.
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The zoo's role will be reviewed by the US Department of Agriculture, which enforces the Animal Welfare Act. An animal protection watchdog group has urged that the zoo face federal fines.
The zoo said that there had been no earlier breaches in Gorilla World's 38-year history and that the previous barrier had passed multiple inspections by the Association of Zoos and Aquariums, which accredits zoos.