Cincinnati - Animal rights activists gathered on Monday for a Memorial Day vigil for the gorilla killed at the Cincinnati Zoo after a 4-year-old boy slipped into an exhibit and a special zoo response team concluded his life was in danger.Anthony Seta of Cincinnati called the 17-year-old western lowland gorilla's death "a senseless tragedy", but said the purpose on Monday wasn't to point fingers, but a tribute to the gorilla named Harambe."People can shout at the parents and people can shout at the zoo," Seta said. "The fact is that a gorilla that just celebrated his birthday has been killed."The gorilla's birthday was May 27, the day before he was shot.There has been an outpouring on social media of people upset about the killing of the member of an endangered species. A Facebook page called "Justice for Harambe" created on Saturday night had drawn wide attention, along with online petitions and another page calling for a June 5 protest at the zoo."I was just trying to make a tribute and it's really taken off from there," Kate Villanueva of Erlanger, Kentucky, said of the "Justice for Harambe" Facebook page.Videos taken by zoo visitors showed the gorilla at times appeared to be protective of the boy, but also dragged him through the shallow moat.TranquilizerThe zoo's 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 the 181kg-plus gorilla didn't appear to be attacking the child, but was in an "agitated situation" and was "extremely strong". He said a tranquilizer wouldn't 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," 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 boy's parents and said they should be held accountable. A Cincinnati police spokesperson said there were no charges being considered. A spokesperson for the family said on Monday they had no plans to make additional comments."I do think there's a degree of responsibility they have to be held to," said Villanueva, a 28-year-old mother of two children. "You have to be watching your children at all times."The People for the Ethical Treatment of Animals (PETA) 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's 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.