• Harambe the gorilla was shot dead when boy fell into an enclosure
• Cincinnati Zoo in Ohio defended the shooting
• Police have launched an investigation into the tragedy
• Boy's parents are receiving death threats
• Two polar bears escaped holding area at zoo in March
The parents of the boy who climbed over a zoo barricade and fell into a gorilla enclosure, leading to the endangered animal's death, have received terrifying threats.
The news comes as police launched an investigation into the weekend tragedy, with records showing the zoo was warned over a serious security breach in March.
Michelle Gregg, 32, and Deonne Dickerson, 37, sparked fury among animal lovers after workers at Cincinnati Zoo in Ohio shot dead Harambe to save their four-year-old son's life on Saturday.
Gregg, a mother of four, broke her silence on Sunday to say people were too quick to judge.
"As a society we are quick to judge how a parent could take their eyes off of their child and 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," she wrote on Facebook.
Now she and her husband, have received countless death threats.
One Twitter user wrote: "How about blaming the f*****g numb-nut parents. Shoot them not 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," another furious tweeter said.
Another said: "Feel for the gorilla. Didn't deserve to get shot. Shoot the s****y mum instead."
Dickerson has a lengthy criminal record. Criminal filings against him stretch over a decade and include burglary, firearms offences, drug trafficking, criminal trespass, disorderly conduct and kidnap.
Police announce investigation
Cincinnati police have launched an investigation into the weekend tragedy, with a federal investigation also planned. Hamilton County Prosecutor Joe Deters' office is looking into what transpired with the gorilla's death.
Afterwards, police will talk with prosecutors about whether charges are warranted, the office said.
Over the weekend, police said no charges were planned. However spokeswoman Tiffaney Hardy said on Tuesday they are still gathering information on what happened.
Zoo warned over March breach
While the zoo has not been charged over the latest disaster, AP reports it was previously warned over a security breach.
Federal reports show an inspector warned the zoo that the public could have been "at great risk for injury, harm or death" on March 16 when two polar bears went through an open den door into a behind-the-scenes service hallway.
At the time, zoo officials said some visitors were moved for safety as the bears were returned to their main holding area. No one was injured.
The federal inspection found that two doors were left open by keepers and there didn't appear to be "a formalised method" for double-checking locks and doors.
The report said animals can be harmed when they access areas not meant for their use, adding: "Surprising the bears in the keeper area could have resulted in human injury or death." It also said the public would have been at great risk if the bears had gotten outside.
The report said the zoo's dangerous-animal response team used tranquilliser darts on the two bears.
A routine inspection April 4-7 that included the gorilla area didn't find any violations, another report said.
Inspections in 2014 found several issues including the need to repair areas where monkeys and horses were housed and a camel that appeared to be badly bothered by flies.
Donald Trump weighs in
Presumptive Republican presidential nominee Donald Trump says the Cincinnati Zoo had little choice but to kill the 17-year-old gorilla.
The billionaire referred to videos showing the animal at times appearing protective of the child, who got away from his mother and entered the gorilla's enclosure.
Trump says watching the gorilla with the boy was "almost like a mother holding a baby", but also noted it showed the gorilla dragging the boy through a shallow moat.
Trump acknowledged that it was "a very tough call" but says the zoo had little choice because the child's life was at stake.
Animal advocates file complaint
Animal protection watchdog group Stop Animal Exploitation NOW has filed a federal complaint accusing the Cincinnati Zoo of negligence in maintaining its gorilla habitat.
Co-founder Michael Budkie said the group is seeking the maximum penalty of $US10,000 ($13,915) for the zoo.
"It's clear that this enclosure is not capable of keeping a four-year-old child out and must violate federal regulations," he said.
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.
Animal behaviour experts say the gorilla would not have harmed the child.
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. He said the zoo remains safe for its 1.6 million annual visitors despite the weekend tragedy.
Anger over Harambe's death sparked more than 400,000 signatures on online petitions on Change.org, some demanding "Justice for Harambe".