Anti-terror police in Turkey have launched raids and arrested 12 people over the New Year's shooting at a nightclub in Istanbul, which left 39 people dead.
Deputy Prime Minister Numan Kurtulmus said authorities had fingerprints and a basic description, and vowed to "speedily" identify the suspect.
Mr Kurtulmus told reporters: "Information about the fingerprints and basic appearance of the terrorist have been found.
"The next step will be to try to identify him as quickly as we can. We hope we will not only find the terrorist but also his connections and those people who gave him support inside and outside the club," he said.
The so-called Islamic State (IS) has also said it was behind the attack.
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.
Mr Kurtulmus confirmed eight arrests. Later in the evening special forces police - backed by a helicopter - launched an operation on a house in the Zeytinburnu district following a tip.
The attacker was not found, but four more arrests were made elsewhere. Turkish police also released new images said to be of the suspect, but without giving any more information about the time or location.
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.
Gunman arrived by taxi
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.
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 said.
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".
About two-thirds 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.