"Police alerted us to a video on Faceook shortly after the livestream commenced and we quickly removed both the shooter's Facebook and Instragram accounts and the video", the social network said. "We are working with social media platforms, who are actively removing this content as soon as they are made aware of an instance of it being posted".
"Our hearts go out to the victims, their families and the community affected by the horrendous shootings in New Zealand", the company added.
"A person involved with the attacks also appeared to post regularly to the "/pol/ - Politically Incorrect" forum on 8chan, a online discussion site known for allowing virtually any content, including hate speech. "Please know we are working vigilantly to remove any violent footage".
Grygiel said it has become commonplace for perpetrators to use social media to gain attention to acts of violence, and that these are often shared on YouTube and other platforms. "The content of the video is disturbing and will be harmful to people to see", he said.
The video's spread underscores the challenge for Facebook even after stepping up efforts to keep inappropriate and violent content off its platform.
"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", Lucinda Creighton, a senior adviser at the Counter Extremism Project, an worldwide policy organization told CNN.
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The videos show the gunman driving to one mosque, entering and shooting randomly at people inside.
But the livestream lasting 17 minutes was shared repeatedly on YouTube and Twitter, and internet platforms were scrambling to remove videos being reposted of the gruesome scene.
PewDiePie, whose real name is Felix Kjellberg, said on Twitter he felt "absolutely sickened" that the alleged gunman referred to him during the livestream.
But Reuters found videos of the shooting on all five platforms up to 10 hours after the attacks, which began at 1345 local time in the city of Christchurch. "This is a case where you're giving a platform for hate".
In August previous year, a shooting at a Madden NFL 19 video-game tournament in Jacksonville, Florida, was captured on live video.
Other violent crimes that have been live-streamed on the internet include a father in Thailand in 2017 who broadcast himself killing his daughter on Facebook Live.
"We are adding each video we to find to an internal data base which enables us to detect and automatically remove copies of the videos when uploaded again", she said in a statement.