It seemed like quick thinking when the Cincinnati Zoo shot to death a gorilla that was manhandling a small boy who had fallen into its enclosure on Saturday afternoon.But soon supporters of animal rights were standing organizing a vigil outside the zoo in remembrance of the gorilla named Harambe, a male weighing more than 420 pounds. Online petitions circulated blaming the mother of the child for negligence. By Monday the chorus of outrage had reached such an intense pitch that the zoo held a news conference to defend itself.Speaking to reporters, zoo director Thane Maynard forcefully rejected the second-guessing of what he called "Monday morning quarterbacks" over the decision to kill the gorilla, one of an endangered species, instead of using a tranquilizer dart.After the 3-year-old boy* breached a barrier to the Gorilla World exhibit and fell into its moat, Harambe seemed to take an intense interest in the child."The child was being dragged around," Maynard said. "His head was banging on concrete. This was not a gentle thing. The child was at risk."Maynard said a tranquilizer would have taken precious minutes to knock out the western lowland gorilla, while the impact of the dart might have startled him, posing an even graver danger.Maynard said the animal was capable of crushing a coconut with his hands. The boy was lucky that Harambe had not done so to him, if only out of carelessness, he said, noting that the gorilla had become agitated and disoriented by screams from the crowd.PREVIOUS COVERAGE: 'Mommy loves you!' Boy rescued after gorilla is shot at zoo Neither the child nor his mother has been publicly identified. But the boy's family, in a statement released through a public relations firm, said that he was doing fine after being treated at a hospital and released on Saturday."We are so thankful to the Lord that our child is safe," the family said. "We extend our heartfelt thanks for the quick action by the Cincinnati Zoo staff. We know that this was a very difficult decision for them, and that they are grieving the loss of their gorilla."The boy's mother, however, has certainly not been at peace. On Twitter, Facebook and other forums, tens of thousands of people have expressed vitriol over her failure to keep the boy out of trouble.By late Monday, an online petition had garnered more than 180,000 signatures calling for her to be investigated by law enforcement and child protective services agencies for possible child neglect. The Cincinnati police said on Monday that they had no such plans.In hundreds of comments on the petition, supporters said Harambe, an innocent animal, had not deserved to die.Shannon Blackmer of Raleigh, N.C., summed up one of the themes: "One woman's negligence cost a primate his life and nearly cost her son his."Outside the zoo on Monday, the gorilla's supporters held placards that read "R.I.P. Harambe," "Because his life mattered" and "In loving memory, Harambe." Inside, flowers were placed at a gorilla statue.Witnesses at the zoo's popular Gorilla World enclosure said the boy had somehow slipped past its wire and wood barriers, then scurried through brush before falling about 10 feet into a small moat.For about 10 minutes, Harambe held onto the boy, at times standing over him in what appeared to be a protective posture, but at others darting across the moat while yanking the child violently by the ankle. A woman could be heard yelling, "Mommy loves you!"On Monday, Maynard defended the enclosure's barriers. He said they had passed regular checks by an accrediting agency and had not been breached since the exhibit opened in 1978."The barriers are safe," he said. "The barriers exceed any required protocols. The trouble with barriers is that whatever the barrier is, some people can get past it."Asked by reporters who was then to blame, Maynard reiterated that the barrier was adequate, then added: "We all need to work to make sure that our families and our kids are safe, whether you're visiting the zoo or you're visiting the shopping mall."He repeatedly declined to blame anyone.Rob Vernon, a spokesman for the Association of Zoos and Aquariums, which accredits zoos, said it expected a report from the Cincinnati Zoo."I would expect that through the process of a review that we may find that we need to have more stringent protocols related to gorilla enclosures, and that's the direction we'll head," he said.Some people have come to the mother's defense.Kevin Figueroa Torres, who set up a Facebook page in support of the mother, said he had learned how slippery small boys could be when he helped run a preschool for four years. People do not get that, he said."It just bothers me," Torres, 23, said in an interview on Monday. "They're acting like kids don't run off, like kids don't do sneaky stuff like that."Brittany Nicely was standing nearby on Saturday when the boy fell into the pen. She said it had all happened very fast. The mother, she said, had not been neglectful."She had three other kids that she was with," said Nicely, 29. "She had a baby in her arms."The boy had made his move and was in the pen in "the blink of an eye," she said.Maynard said on Monday that the zoo was going through a tough time, but that it had also received plenty of support."We've heard from thousands of people around the world," he said, "colleagues all the way to Jane Goodall, zoo directors from all over the world, with both sympathy and with support for a difficult decision."* Editor's note: This story and its headline have been edited to reflect a correction. The boy who fell into the enclosure is 3 years old.