A gunman opened fire on New Year revelers at a packed nightclub on the shores of Istanbul's Bosphorus waterway on Sunday killing at least 39 people, including many foreigners, then fled the scene.
Some people jumped into the Bosphorus waters to save themselves after the attacker opened fire at random in the Reina nightclub just over an hour into the new year. Officials spoke of a single attacker but some reports, including on social media, suggested there may have been more.
The attack shook NATO member Turkey as it tries to recover from a failed July coup and a series of deadly bombings in cities including Istanbul and the capital Ankara, some blamed on Islamic State and others claimed by Kurdish militants.
Security services had been on alert across Europe for new year celebrations following an attack on a Christmas market in Berlin that killed 12 people. Only days ago, an online message from a pro-Islamic State group called for attacks by "lone wolves" on "celebrations, gatherings and clubs".
The Hurriyet newspaper cited witnesses as saying the attackers shouted in Arabic as they opened fire at Reina.
"We were having fun. All of a sudden people started to run. My husband said don't be afraid, and he jumped on me. People ran over me. My husband was hit in three places," one club-goer, Sinem Uyanik, told the newspaper.
"I managed to push through and get out, it was terrible," she said, describing seeing people soaked in blood.
The incident bore echoes of an attack by militant Islamists on Paris's Bataclan music hall in November 2015 that, along with assaults on bars and restaurants, killed 130 people.