Hours after the attack, however, copies of the gruesome video continued to appear on Facebook, YouTube and Twitter, raising new questions about the companies' ability to manage harmful content on their platforms.
Facebook is "removing any praise or support for the crime and the shooter or shooters as soon as we're aware," Garlick said.
New Zealand police asked social media users to stop sharing the purported shooting footage and said they were seeking to have it taken down.
CNN is choosing not to publish additional information regarding the video until more details are available.
Tech firms 'do not see this as a priority'
Friday's video reignites questions about how social media platforms handle offensive content: Are the companies doing enough to try to catch this type of content? How fast should they be expected to remove it?
"While Google, YouTube, Facebook and Twitter all say that they're cooperating and acting in the best interest of citizens to remove this content, they're actually not because they ' re allowing these videos to reappear all the time, "said Lucinda Creighton, a senior adviser at the Counter Extremism Project, an international policy organization.
Facebook's artificial intelligence tools and human moderators were apparently unable to detect the livestream of the shooting. The company says it was alerted to it by New Zealand police.
"The tech companies basically do not see this as a priority, they wring their hands, they say this is terrible," Creighton said. "But what they're not doing is preventing this from reappearing."
John Battersby, a counter-terrorism expert at Massey University of New Zealand, said the country had been spared mass terrorist attacks, partly because of its isolation. Social media had changed that.
"This fellow live streamed the shooting and his supporters have cheered him on, and most of them are not in New Zealand," he said. "Unfortunately once it's out there and it's downloaded, it can still be (online)," he added.
The spread of the video could inspire copycats, said CNN legal enforcement analyst Steve Moore, a retired supervisory special agent for the FBI .
"What I would tell the public is this: Do you want to help terrorists? Because if you do, sharing this video is exactly how you do it," Moore said.
"Do not share the video or you are part of this," he added.
Hadas Gold, Donie O'Sullivan, Samuel Burke and Paul Murphy contributed to this report.