Thirty-five people were killed and 40 hurt Sunday when at least one gunman reportedly wearing a Santa outfit stormed a popular Istanbul nightclub and sprayed bullets at revellers celebrating the New Year.
Here is what we know about the massacre, the latest to rock Turkey after a bloody 2016.
At least one gunman shot dead a policeman and a civilian at the entrance to the Reina nightclub and then went on a shooting rampage inside, Istanbul governor Vasip Sahin said.
According to some witnesses cited by the Dogan news agency, the attackers were "speaking Arabic".
Sahin wasted no time branding the bloodshed a "terror attack", the latest to strike Turkey after a wave of assaults by Islamic State jihadists and Kurdish guerrillas.
According to NTV television, special forces officers were still searching the premises.
TV pictures showed ambulances ferrying the wounded from the scene as police cordoned off the area.
The attack took place at the swanky Reina nightclub in the Ortakoy district on the banks of the Bosphorus on the European side of the city.
There were reportedly as many as 700 people dancing to celebrate the New Year, which chimed in barely an hour before the attack.
The club is known as one of the most exclusive nightspots in the city and it is notoriously hard to get past the bouncers, who seek out only the best dressed.
Television pictures showed shellshocked revellers dressed up to the nines - men in suits and women in cocktail dresses - emerging dazed from the scene.
The attack sparked mass panic, with some diving into the Bosphorus Strait between Europe and Asia to escape the bullets. Rescuers were battling to salvage them from the water.
After a bloody 2016, the authorities were on their guard and at least 17,000 police officers were deployed in the city for the New Year festivities.
Turkey has endured bomb attacks at an airport, a suicide bombing at a wedding and an attack near a top football stadium last year.
The attacks have been blamed either on Kurdish militants or the Islamic State jihadist extremist group.
Last month, the Russian ambassador was shot dead at an Ankara art gallery by an off-duty policeman shouting "Allahu Akbar" (God is greatest) and "Don't forget Aleppo".
The Turkish army is waging a four-month incursion in Syria to oust the IS group and Kurdish militants from the border area.
Rebels from the outlawed Kurdistan Workers' Party (PKK) and what is considered its radical offshoot the Kurdistan Freedom Falcons (TAK) have claimed a spate of attacks since the collapse of a ceasefire in the summer of 2015.
National Security Council spokesman Ned Price said the United States condemned in the strongest terms the "horrific terrorist attack".
"That such an atrocity could be perpetrated upon innocent revellers, many of whom were celebrating New Year's Eve, underscores the savagery of the attackers." Price added: "We reaffirm the support of the United States for Turkey, our NATO ally, in our shared determination to confront and defeat all forms of terrorism.
NATO Secretary General Jens Stoltenberg tweeted: "Tragic start to 2017 in #Istanbul. My thoughts are w/ those affected by the attack on people celebrating New Year and w/ the Turkish people." The sentiment was echoed by the EU's foreign policy chief Federica Mogherini.
"2017 starts with an attack in #Istanbul. Our thoughts are with victims and their loved ones. We continue to work to prevent these tragedies." burs-txw/ric .