Animal rights activists have staged a vigil for the gorilla killed at Cincinnati Zoo after a four-year-old boy slipped into his enclosure.
A special zoo response team concluded the boy's life was in danger so killed the 17-year-old western lowland gorilla.
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.
Zoo director Thane Maynard said its dangerous animal response team that includes full-time animal keepers, veterinarians and security staff made the right call to kill the gorilla.
He said on Saturday that although the gorilla did not appear to be attacking the child, he was in an "agitated situation" and was "extremely strong". He said a tranquilizer would not have immediately felled the gorilla, leaving the child in danger.
"We are heartbroken about losing Harambe, but a child's life was in danger and a quick decision had to be made," Mr Maynard said in statement on Sunday.
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.