Facebook, YouTube and Twitter are struggling to halt the spread of horrific footage that appears to show a massacre at a mosque in New Zealand as it was taking place. Dozens of people were killed Friday in shootings at two mosques in the city of Christchurch.

One of the shooters appears to have livestreamed the attack on Facebook (FB). The disturbing video, which has not been verified by CNN, ran for nearly 17 minutes and purportedly shows the gunman walking into a mosque and opening fire.

"New Zealand Police alerted us to a video on Facebook shortly after the livestream commenced and we quickly removed both the shooter's Facebook and Instagram accounts and the video," Mia Garlick, Facebook's director of policy for Australia and New Zealand, said in a statement.

Read more: CNN