The Islamic State in Iraq and Syria claimed responsibility on Monday for a New Year's Day mass shooting in a packed Istanbul nightclub that killed 39 people, an attack carried out by a lone gunman who remains at large. 

It described the Reina nightclub, where many foreigners as well as Turks were killed, as a gathering point for Christians celebrating their "apostate holiday." The attack, it said, was revenge for Turkish military involvement in Syria. 

Ontario woman among those killed in Istanbul attack
PHOTOS | Turkey mourns after New Year's revellers gunned down