Police may bring criminal charges over a Cincinnati Zoo incident in which a gorilla was killed to rescue a four-year-old boy who had fallen into its enclosure, a prosecutor said.
The death of Harambe, a 200 kilogram gorilla, also prompted the animal rights group Stop Animal Exploitation Now to file a negligence complaint on Tuesday against the zoo with the US Department of Agriculture. The group is seeking the maximum penalty of $US10,000 ($A13,833).
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."
Mounting outrage over Saturday's killing of the Western lowland silverback, an 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."
Cincinnati police are taking a second look at possible criminal charges in the incident after initially saying no one was charged. There was no indication of whether the investigation would focus on the zoo or the child's parents.
"Once their investigation is concluded, they will confer with our office on possible criminal charges," Hamilton County Prosecutor Joseph Deters said in a statement.
Witnesses said the child had expressed a desire to get into the enclosure and climbed over a one-metre barrier, falling 4.6 metres into a moat.
Zookeepers took down the 17-year-old ape after he violently dragged and tossed the child, officials said.
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 in a CNN interview 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?'"