Police are investigating the circumstances which led to a gorilla being shot dead at Cincinnati Zoo to protect a young boy who had entered its cage.
Officers said over the weekend no charges were planned but now it has emerged they are still gathering information and charges are possible.
It comes after more than 300,000 people signed a petition calling for the parents of the four-year-old concerned to be probed.
Others have called for the zoo to be held responsible for the death of the Western Lowland Gorilla, named Harambe, on Saturday.
The 17-year-old animal, whose species is listed as endangered, was shot minutes after dragging the child through shallow water and up a rock wall.
Tranquillisers were not an option because they could have a delayed effect and may have agitated the gorilla further, zoo officials said.
The petition, “Justice For Harambe”, accuses the boy’s parents of “negligence”, although they have denied this.
His mother, Michelle Gregg, asked people not to judge her and her partner Deonne Dickerson because “accidents happen”.
Writing on her Facebook page, she added: “If anyone knows me I keep a tight watch on my kids. Accidents happen but I am thankful that the right people were in the right place today.”
Ms Gregg said her son was recovering from concussion and a few scrapes.
A spokeswoman for the US Department of Agriculture’s animal and plant health inspection service, said it has not begun an inquiry yet but would be checking for any violations of the Animal Welfare Act.
Cincinatti Zoo’s director, Thayne Maynard, insisted it is safe for the 1.6 million people who visit it every year but said a review is underway to see if any improvements can be made.
Early in April a routine inspection found the gorilla enclosure did not breach safety regulations.
But the zoo was severely reprimanded in March when two polar bears went through an open den door into a service hallway.
Nobody was hurt but an inspector warned members of the public could have been “at great risk for injury, harm or death”.
Meanwhile, the decision to kill Harambe has won the backing of Republican presidential hopeful Donald Trump.
He acknowledged it was “a very tough call” but said the life of the child had to come first.