The attacker, who remains at large, shot dead a police officer and a civilian at the entrance to the exclusive Reina nightclub on Sunday. He then opened fire with an automatic rifle inside, reloading his weapon half a dozen times and shooting the wounded as they lay on the ground.
In a statement claiming the attack on Monday, Islamic State described the club as a gathering point for Christians celebrating their "apostate holiday" and said the shooting was revenge for Turkish military involvement in Syria.
"The assailant has experience in combat for sure ... he could have been fighting in Syria for years," one security source told Reuters, saying that he was likely to have been directed in his actions by the jihadist group.
The Haberturk newspaper said police investigations had revealed that the gunman had entered Turkey from Syria and went to the central city of Konya in November, traveling with his wife and two children so as not to attract attention.