The director of Cincinnati Zoo's has insisted the decision to kill a gorilla after a boy entered its enclosure was the right one.
Thane Maynard told a news conference that the four-year-old boy's life was in danger when he strayed into the moat at Gorilla World.
He said the 420lbs gorilla called Harambe was agitated and disoriented and acting erratically.
He added the western lowland gorilla was extreme strong and could crush a coconut in its hand.
A witness reported hearing the boy tell his mother he wanted to get in the water at the enclosure on Saturday afternoon. She said no, but the boy entered a moat in the gorilla exhibit anyway.
Mr Maynard said the zoo is reviewing security measures to ensure both visitors and animals are safe.
He revealed staff have received messages of support and condolences from around the world after the decision to shoot the gorilla.
He acknowledged there were also critics of the zoo's decision, describing the situation as "very emotional".
"Not everyone shares the same opinion and that's OK," Mr Maynard said.
"But we all share the love for animals."
Earlier animal rights activists staged a vigil for Harambe at the zoo.
Anthony Seta, of Cincinnati, called the animal's death "a senseless tragedy", but said the purpose of the vigil was not to point fingers but to pay tribute to the gorilla, named Harambe.
"People can shout at the parents and people can shout at the zoo," Mr Seta said.
"The fact is that a gorilla that just celebrated his birthday has been killed."
The gorilla's birthday was on May 27, the day before he was shot.
There has been an outpouring on social media of people upset about the killing.
A Facebook page called Justice for Harambe has drawn wide attention, along with online petitions and another page calling for a protest at the zoo on June 5.
Videos taken by zoo visitors showed the gorilla at times appeared to be protective of the boy, but he also dragged him through the shallow moat.
The boy was taken to Cincinnati Children's Hospital Medical Centre for treatment and was released on Saturday night. His parents said in a statement on Sunday that he was "doing just fine".
Many social media commenters have criticised the parents and said they should be held accountable.
Villanueva, a 28-year-old mother-of two, said: "I do think there's a degree of responsibility they have to be held to. You have to be watching your children at all times."
A Cincinnati police spokesman said there are no charges being considered. A spokeswoman for the family said they have no plans to make additional comments.
The People for the Ethical Treatment of Animals released a statement from its primatologist Julia Gallucci saying the zoo should have had better barriers between humans and gorillas.
"This tragedy is exactly why Peta urges families to stay away from any facility that displays animals as sideshows for humans to gawk at," the statement said.
The zoo said it is the first such spectator breach at Gorilla World since it opened in 1978 and that the exhibit undergoes regular outside inspections.
The zoo said earlier this year it plans to expand the exhibit.
Gorilla World remained closed on Monday.