Turkish police officers block the road leading to the scene of an attack in Istanbul, early Sunday, Jan. 1, 2017. (AP)
Turkish police detained several new suspects on Thursday in fresh raids over the nightclub attack that killed 39, as authorities tightened borders to prevent the fugitive killer from escaping.
A top official said the attacker was likely a Turkic Uighur and reports have indicated the authorities are looking into the possible existence of a cell, also including other jihadists from Central Asia.
But with the prospect of a dangerous trained killer on the run in the city adding to tensions in Istanbul after a spate of attacks, the authorities have still not caught the individual.
In the early hours of Sunday, a gunman stormed the swanky Reina nightclub on the shores of the Bosphorus in Istanbul and sprayed 120 bullets at terrified partygoers celebrating New Year.
Of the 39 dead, 27 were foreigners including citizens from Lebanon, Saudi Arabia, Israel, Iraq and Morocco.
The Islamic State group (IS) took responsibility for the massacre in a statement on Monday, marking the first time it has issued a clear and undisputed claim for a major attack inside Turkey.
The extremist group said it was a response to Ankara's ambitious military operation against it in northern Syria, where Turkish armed forces are supporting opposition fighters retaking territory from IS.
Special forces detained several people suspected of links with the attack on the outskirts of Istanbul on Thursday, state-run news agency Anadolu said.
Authorities also tightened Turkish land borders, Dogan news agency reported, over fears the killer planned to flee the country.
The agency said checkpoints would be set up to search all vehicles and people leaving the country at border crossings in Edirne, western Turkey, which has a land border with Greece and Bulgaria.
Turkish Foreign Minister Mevlut Cavusoglu said on Wednesday that the attacker had been identified but did not give a name or further details.
Deputy Prime Minister Veysi Kaynak told A Haber broadcaster on Thursday that the attacker was "probably" of Uighur origin as he sought to play down fears that the gunman would escape Turkey.
Most Uighurs, an eastern Turkic group, live in the Xinjiang region of China, although there are also significant populations in ex-Soviet Central Asian states.
Previous reports had said the killer could be from Kyrgyzstan or Uzbekistan. Kaynak said airports had also taken important measures to ensure the killer did not escape.
Responding to some reports in local media that there was a second gunman, Kaynak said security forces were "assessing all probabilities" but that the shots were fired from one gun.
He said investigations continued into whether the gunman had help inside the club and what kind of assistance he had to prepare the attack.
Kaynak described the massacre as "sophisticated and well planned", suggesting the gunman is part of a "well formed cell".
Anadolu did not say how many were arrested during the raids in Istanbul but claimed those detained were of Uighur origin.
Police reportedly got a tip-off that individuals linked to the killer were hiding in a housing complex in the district.
Excluding the latest arrests, at least 36 had been detained in the probe by Wednesday, but the gunman himself remains on the run after giving police the slip in the aftermath of the massacre.
Authorities had on Tuesday detained 20 adults -- along with 20 of their children -- from three families in the Aegean city of Izmir.
They moved to Izmir on December 10 from the central city of Konya, where they are suspected of having been in touch with the killer during his stay there.
Police are investigating possible links to IS of these suspects, who include Uighurs, Kyrgyz citizens and suspects from the Russian Caucasus region of Dagestan.
The latest attack comes after Turkey suffered a bloody year in 2016 when multiple bombings blamed on Kurdish militants and IS jihadists killed hundreds of people.
Turkey is also still reeling from a failed July 15 coup blamed by the government on the US-based Islamic preacher Fethullah Gulen that has been followed by a relentless purge of his alleged supporters from state institutions.
Parliament on Tuesday extended a controversial state of emergency in place since the coup -- and which has seen over 41,000 people arrested -- by another three months to April 19.
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