A wildlife expert claims killing Harambe the gorilla was "the only decision" for the response team at Cincinnati Zoo.
The 17-year-old gorilla was shot dead after a 10-minute incident after a child fell into his enclosure.
The boy, whose name is yet to be confirmed, escaped with only minor injuries.
He is reported to have crawled through a barrier before falling 12 feet into Harambe's pen.
Footage from the incident has been seen online, with many people claiming the gorilla didn't mean to harm the boy.
Charlotte Macdonald, director of life sciences at Twycross Zoo, believes shooting Harambe was the only course of action.
"We're only seeing 40 seconds of what was a 10-minute interaction," she tells Newsbeat.
"So the animal response team did take the time to test the situation and assess his behaviour. We don't know what happened outside the little clips of video we have seen.
Harambe the gorilla
"But from what I've seen, I can see why they took the decision they did. I think it was the only decision they could make.
"They take time to work. They take 5-10 minutes to work in a normal situation."
Charlotte also claims the stress of the dart could have resulted in an aggressive reaction from Harambe, most likely against the child in the enclosure.
"In this situation, where the animal is already stressed, tranquilisers would take even longer and in fact, he may have responded quite negatively to being darted," she says.
Bronze statue of a gorilla and her baby outside the Cincinnati Zoo's Gorilla World exhibit
"He would have felt the needle going in at a high velocity. He would have felt that.
"It may have angered him and he may have responded quite badly to that."
The killing has led to anger from animal lovers, but after viewing the footage, Charlotte says Harambe's actions weren't necessarily friendly, despite gorillas being peaceful creatures.
"He's using the child in part of a display," she tells Newsbeat.
Gladys Porter Zoo facilities director Jerry Stones speaks to members of the media about his memory of Harambe
Image caption
Gladys Porter Zoo facilities director Jerry Stones helped raise Harambe when he was little
"The display is because he's feeling a bit cornered and a bit threatened and he's not quite sure what's going on.
"Gorillas are usually quite a gentle species, even Silverbacks, but in an unknown situation where he's a little bit frightened or nervous he'll be very unpredictable.
"His sheer strength could mean he'd hurt the child even if he wasn't meaning to. So even without being overtly aggressive, he could have caused grave injury to that child."