So-called Islamic State says it was behind the New Year attack on a Turkish nightclub that killed 39 people.
The group said in a statement it was carried out by “a heroic soldier”.
At least 600 revellers were celebrating in the early hours of Sunday at Istanbul’s Reina nightclub when the gunman began firing indiscriminately.
The gunman is still believed to be at large and a manhunt is under way. Police said they had made eight arrests so far.
IS has been blamed for recent attacks in Turkey, which is taking military action against the group in neighbouring Syria.
The militant group has already been linked to at least two attacks in Turkey last year.
The IS statement accused Turkey of shedding the blood of Muslims through “its air strikes and mortar attacks” in Syria.
Turkey launched a military operation in August aimed at pushing back IS and Kurdish forces, with some of the most intensive recent fighting against IS round the northern town of al-Bab.
Turkey has also been a key player with Russia in negotiating a truce between moderate rebel forces and the government.
More details of the nightclub attack have been emerging. The gunman, who fired up to 180 bullets, arrived by taxi before rushing through the entrance with a long-barrelled gun he had taken from the boot of the car.
The attacker fired randomly at people in an assault lasting seven minutes, starting with a security guard and a travel agent near the entrance. Both were killed.
The gunman is reported to have removed his overcoat before fleeing during the chaos.
Turkish media reports quote police sources as saying he may have been from Uzbekistan or Kyrgyzstan.
Police are investigating whether he belongs to an IS cell blamed for an attack in June on Ataturk Airport in Istanbul.
President Recep Tayyip Erdogan accused groups such as IS of trying “to create chaos”.
“They are trying to… demoralise our people and destabilise our country,” he added.
Turkey suffered a bloody 2016 with a series of attacks, some carried out by Kurdish militants.
But a day before the IS claim, the banned Kurdistan Workers’ Party (PKK) was quick to distance itself from the nightclub attack, saying it would “never target innocent civilians”.
Twenty-seven of those killed were foreign, according to local media, among them citizens from Israel, Russia, France, Tunisia, Lebanon, India, Belgium, Jordan and Saudi Arabia.
Bollywood director and producer Abis Rizvi was one of those who died, Indian media report.