Deters said an investigation by the Cincinnati Police Department determined that "none of the witnesses interviewed described the [boy's] 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 3-year-old was able to climb into the gorilla enclosure."
He added: "I am very sorry about the loss of this gorilla, but nothing about this situation rises to the level of a criminal charge."
Video obtained by ABC News shows the 450-pound gorilla, named Harambe, dragging the small boy through water in the enclosure on May 28. The boy was in the enclosure for 10 minutes before the gorilla was shot in the head and killed by zoo staff. The boy was taken to Cincinnati Children's Hospital and released without life-threatening injuries.
Deters said the boy suffered only minor scrapes and bruises. "I don't know how he wasn't hurt," he said.
"There was nothing that the mother could have done" in the incident, Deters said. "I just think we're lucky that the zoo did what they had to do."
Members of the boy's family said in a statement they were "very pleased with this decision. This is one more step in allowing us to put this tragic episode behind us and return to our normal family life. We extend thanks to all of those who have been praying for us and who have supported us through this trying ordeal."
ABC News' Michael DelMoro contributed to this report.