One of the New Zealand attackers streamed live on Facebook as he murdered dozens of worshippers at a mosque.
The footage, lasting more than 16 minutes, was posted on a page belonging to Brenton Tarrant but was quickly deleted by Facebook.
Apparently filmed from a head camera, it shows a man in combat gear driving to the Al Noor Mosque in central Christchurch. After parking his vehicle he picks up one of a number of rifles inside and walks to the mosque, starting shooting as he approaches the door.
The footage shows the man spending several minutes inside, reloading his rifle several times. At one point he leaves the mosque to fetch another weapon from his car, firing on people on the street before returning and systematically shooting into the bodies.
He then leaves, shooting dead another passerby as she pleads for help before driving off in his car, firing another gun through his window.
Police said they were working to have the footage removed from the internet.
Facebook said they had removed the profiles concerned and were deleting any messages of support for the attacks.