A gunman opened fire on New Year's Eve revellers at a packed nightclub on the shores of Istanbul's Bosphorus waterway on Sunday killing at least 39 people, including many foreigners.
The gunman escaped after the massacre.
Authorities have initiated a massive security operation to track down the assailant and his conspirators in the city.
The gunman killed a policeman and another man outside the Reina club in the early hours of 2017, before entering and firing with an automatic rifle at partygoers inside.
The killings plunged the city into mourning.