Around 600 people are said to have been inside Istanbul's popular Reina nightclub when a gunman opened fire less than an hour into 2017.
Some are reported to have jumped into the Bosphorus Strait to escape the gunfire.
Turkish Prime Minister Binali Yildirim says the attacker escaped in the confusion, leaving behind evidence.
"He left his gun at the crime scene and fled, taking advantage of the chaos. Police forces are working on the case. All options are considered. They are working meticulously."
The Turkish Interior Ministry says more than a dozen victims are yet to be identified.
Suleyman Ozcan says he fears for his younger brother, who had been working at the nightclub for just two days.
"I ran here upon hearing the news. It's terrible that I failed to contact him. I could not reach him by phone and cannot even figure out whether he's alive or dead. No word of him. There is no word of him."
Foreign nationals were among the victims, including citizens of Saudi Arabia, Morocco, Lebanon, Libya and Israel.
The father of a 19-year-old Israeli woman says he urged his daughter not to travel to Turkey.
"I told her the security situation in Turkey isn't good and that it's very dangerous but she insisted on going, because her friends wanted to go as a group. The security situation is like this all over the world. She should not have gone there."
The New Year's Eve attack follows a series of deadly bombings in cities including Istanbul and the capital Ankara.
Last month saw attacks on Turkish police and army officers.
Those incidents were blamed on Kurdish separatists, who are fighting against the government in the country's south-east.
Turkey has also come under attack from followers of the so-called Islamic State group.
Prime Minister Binali Yildirim says Turks will not be subdued by terrorism.
"Terror cannot intimidate us. We will intimidate terror. We will continue to fight against it. Our biggest insurance is to see our people standing in solidarity and supporting our resolve. As long as this solidarity continues, there is nothing we can't overcome."
