Mounting outrage over Saturday's killing of the endangered species sparked criticism of both the zoo and the child's parents.
The parents of a 3-year-old boy who fell into a gorilla enclosure in Cincinnati, prompting the killing of the endangered animal in order to rescue the child, are the target of an investigation into the incident, police said on Tuesday.
Investigators are focusing their attention on the actions of the parents and family that led up to the incident over the weekend and not the operation or safety of the Cincinnati Zoo, where officials fatally shot Harambe, a 450-pound (200-kg) gorilla, the Cincinnati Police Department said a statement.
"We are closely reviewing the facts of the case," the department said in a Tweet.
Cincinnati police are taking a second look at possible criminal charges in the incident after initially saying no one was charged.
"Once their investigation is concluded, they will confer with our office on possible criminal charges," Hamilton County Prosecutor Joseph Deters said in a statement.
Police on Tuesday also verified the age of the child to be 3 years old. Earlier reports said the boy was 4 years old.
Witnesses said the child had expressed a desire to get into the enclosure and climbed over a 3-foot (1-meter) barrier, falling 15 feet (4.6 m) into a moat. Zookeepers took down the 17-year-old the Western lowland silverback after he violently dragged and tossed the child, officials said.
Mounting outrage over Saturday's killing of the endangered species sparked criticism of both the zoo and the child's parents. Online petitions at change.org drew more than 500,000 signatures demanding "Justice for Harambe."
The death of the gorilla also prompted the animal rights group Stop Animal Exploitation Now to file a negligence complaint on Tuesday against the zoo with the U.S. Department of Agriculture. The group is seeking the maximum penalty of $10,000.
The group said in its complaint letter that the child's ability to get past the barrier was proof the zoo was negligent and should be fined for a "clear and fatal violation of the Animal Welfare Act."
The boy's mother said on Facebook that the boy suffered a concussion and scrapes but was otherwise fine.
Thane Maynard, director of the Cincinnati Zoo & Botanical Gardens, on Monday stood by the decision to shoot Harambe, saying he was not simply endangering the child but actually hurting him.
Zoo officials were not immediately available for comment on either the negligence complaint or the police investigation but said on Monday the exhibit was safe and exceeded required protocols.
The Gorilla World exhibit has been closed since the incident and will reopen on Saturday.
Looking at the incident through Harambe's eyes, his former caretaker, Jerry Stones, said that the breach of his habitat was likely confusing.
"Here is this animal that has this strange thing in his house," Stones said on CNN. "He knew what adult people were but he'd never been around children. It smells similar, it looks similar but 'What is it? Do I play with it? Am I supposed to be afraid of it? What do I do?'"
Even Republican presidential candidate Donald Trump jumped into the fray at a news conference, saying, "The way he held that child, it was almost like a mother holding a baby ... It was so beautiful to watch that powerful, almost 500-pound gorilla, the way he dealt with that little boy. But it just takes one second ... one little flick of his finger."
In the wild, adult male silverbacks such as Harambe are leaders of groups of gorillas known as troops. They develop the silver patch on their coats as they mature.