The parents of a four-year-old boy who climbed into a gorilla enclosure at Cincinnati Zoo, leading to the death of the animal, have been receiving threats.
Michelle Gregg and Deonne Dickerson have been the target of online animal lovers who are furious staff at the zoo had to shoot dead a Western lowland silverback gorilla named Harambe to save their son's life.
Among the backlash, some Twitter users have been even suggesting the zoo staff should have instead shot Ms Gregg and her husband instead of the gorilla.
"I'm pro animals. Take better care of your 3yr old F*****G KID you lazy son of a b***h. Shoot the parents first then the gorilla. Rant over. [sic]"
Meanwhile, other animal rights activists have filed a federal complaint accusing the Cincinnati Zoo of negligence in maintaining its gorilla habitat after an ape was killed in order to rescue a boy who had fallen into its enclosure.
The complaint, which was filed with the US Department of Agriculture seeks the maximum penalty of US$10,000 (NZ$14,888) for the zoo, said Michael Budkie, co-founder of Stop Animal Exploitation Now on Tuesday.
"It's clear that this enclosure is not capable of keeping a four-year-old child out and must violate federal regulations," he said.
Zoo officials were not immediately available for comment but said on Monday (local time) the exhibit was safe and exceeded required protocols.
Tanya Espinosa, a spokeswoman for the USDA's Animal and Plant Health Inspection Service, said the complaint had been received but that an investigation had not yet been opened.
The USDA regulates zoos, circuses and marine mammal parks under the Animal Welfare Act, the only federal law that requires a minimum standard of humane treatment and care for animals in research, exhibition, transport and by dealers.
Anger over Harambe's death sparked more than 400,000 signatures on online petitions at Change.org, some demanding "Justice for Harambe" and urging police to hold the child's parents accountable.
On Monday, Thane Maynard, director of the Cincinnati Zoo & Botanical Gardens, said a one metre barrier around the gorilla enclosure was adequate, even though the boy was able to climb over it and fall in.
"The trouble with barriers is that whatever the barrier, some people can get past it. ... No, the zoo is not negligent," he said.
The boy's mother on Facebook said he suffered a concussion and scrapes but was otherwise fine.
Maynard stood by the decision to shoot the gorilla, saying he was not simply endangering the child but actually hurting him.
Zoo warned over security breach in March
According to reports from AP, Cincinnati Zoo had also been previously warned in March after a security breach.
Federal reports allegedly show an inspector warned the zoo that the public could have faced "great risk for injury, harm or death" on March 16 when two polar bears went through an open door and into a service hallway.
The incident meant some visitors were moved for safety reasons and the bears were then returned to the holding area.
Nobody was injured, however, AP reported the accident occurred because two doors had been left open by keepers and there wasn't a formalised method of checking locks and doors.
No charges have been laid for either instance, but police have launched a probe into the gorilla's death to work out the circumstances leading up to the killing.