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, discharging some 180 bullets.
The gunman is still at large and a manhunt is under way. Police say they have 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 around 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.
Turkey’s Deputy Prime Minister Numan Kurtulmus said on Monday that the nightclub attack was a “message” against Turkey’s operations in Syria but that they would not be affected.
More details of the nightclub attack have been emerging. The gunman 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.
Kyrgyzstan’s foreign ministry said the reports that the suspect might be Kyrgyz were “doubtful” but that it would carry out checks.
Mr Kurtulmus said Turkish authorities were still working to identify the attacker.
“Information about the fingerprints and basic appearance of the terrorist have been found,” he said.
Police are investigating whether he belongs to an IS cell blamed for an attack in June on Ataturk Airport in Istanbul.