The Turkish authorities have been conducting a manhunt for the assailant, who was able to escape the scene of the shooting and was still at large on Tuesday. The attack, which took place at the Reina nightclub in Istanbul's Ortakoy district, was the first on Turkish soil to have been formally claimed by the "Islamic State" group.

Turkey's prime minister, Binali Yildirim, said on Tuesday that a state of emergency in place in Turkey since July "may be extended." Dozens of people injured in the attack are still receiving treatment in hospitals across the city.

Police have so far arrested 20 people in relation to the attack, including two foreign nationals detained Tuesday afternoon in Istanbul airports.

The Turkish police originally leaked information to the press indicating a 28-year-old Kyrgyz national was the prime suspect, but later established that the man in question was not involved in the attack.

CCTV footage

The investigation in search of the perpetrator is ongoing and is focused on Central Asian and North Caucasus nationals in Turkey.

The assailant's attack on the front of the nightclub was captured by camera number 43 of the club's security system. The club's bouncers are seen crouching down and fleeing inside.

A large man in a red jacket cowers in the corner of the club's entrance as shots coming from the right of the frame appear to ricochet off the building. Another man dives over the rail surrounding the club's porch as the attacker appears from the right. A dog scuttles off away from the attacker, who is hunched over an assault rifle that he moves sharply and precisely between the two men as he fires.

"A terrorist with a long range weapon came to the nightclub, he killed a police officer waiting in front, then shot a citizen and went inside," said Vasip Sahin, the governor of Istanbul in a statement early on Sunday morning.

"He rained bullets brutally and mercilessly over innocent people who were only there to celebrate the New Year," Sahin said.

Once he had entered the club, the assailant moved to the first floor and began firing on the patrons before returning to the ground floor and continuing the attack. Police believe that the assault lasted seven minutes in total, after which the assailant was able to change his clothes and escape.

"I saw the shooting so I went to the toilet. We were eight or nine people hiding in one cubicle. I was just thinking it could be me too," said Tuvana Tugsaval, a nightclub employee.

IS claims attack for the first time

The "Islamic State" is widely considered by the Turkish authorities and expert analysts to have conducted a string of attacks inside Turkey, beginning with a slew of bombings targeting the Kurdish People's Democratic Party's (HDP) offices and rallies in mid 2015.

However the New Year's Day attack is the first to have been officially claimed by the group.

"One of the heroic caliphate soldiers, using an assault rifle and grenades," the IS statement reads, "attacked an idolatrous Christian celebration" as part of the fight "waged by Islamic state against protectors of the cross in Turkey."

The group refers explicitly to Turkey's military operations in Syria as a motivation for the attack. "[This attack will] teach the apostate Turkish government that the Muslim blood it spills with its bombs and its guns will, with god's permission, start a raging fire in its own lands," the statement reads.

Anti-IS raids

Turkish police had conducted anti-IS sweeps across the country just before the attack and the Turkish army has recently taken part in airstrikes on the IS-held town of al-Bab in northern Syria.

Conservative and religious elements inside Turkey spent much of the preceding week denigrating New Year's celebrations as impious and foreign. Several conservative newspapers, including dailies "Yeni Akit" and "Yeni Safak," carried prominent articles on New Year's Eve against the celebrations.

"Turkish security raided and exposed many Turkish national IS cells and sympathizers in recent months. With the [Turkish military intervention] Euphrates Shield in Syria and recent security operations inside Turkey, IS lost the opportunity to shell Turkish border cities or to use its Turkish national hidden cells," said Omer Behram Ozdemir of Sakarya University Middle East Institute and an expert on jihadist recruitment in Turkey.

"Using foreign nationals, who likely have military experience from Syria/Iraq, in attack against Turkey may be a new strategy from IS to hurt Turkey," Ozdemir told DW.

"The IS leader Abu Bakr al-Baghdadi and their new spokesman, Abu Hassan al-Muhajir, are now directly targeting Turkey in their speeches, and call their sympathizers to target Turkish interests all over the world," he said.

'Cream of IS'

According to Ahmet Yayla of George Mason University, an expert on Islamic State and the co-author of Isis Defectors: Inside Stories of the Terrorist Caliphate, the identity of the assailant as a Central Asian may provide important clues regarding IS strategy in Turkey.

"In my experience interviewing IS defectors, members of IS's Caucasus emirates are considered the cream of the force, or the special forces of IS fighters - they are the most reliable forces IS has," Yayla told DW.

"In the past, IS attacks in Turkey have generally been suicide bombings that require little training: they send them with a button and they push it. But this attack required training and skills: the assailant was very comfortable killing people, standing there, keeping his cool, changing the magazines and still shooting."

Yayla points out that the attack succeeded despite a heightened police presence following the assassination of the Russian ambassador in Ankara.  A special police operation in Istanbul has seen 35,000 police officers stationed around the city for the past two weeks.

"Before the al-Bab campaign that Turkey is waging right now in Syria, Turkey and IS were not really in all-out war against one another, but now they are. IS has taken the gloves off with Turkey as a result of this and Turkey's cooperation with Russia," Yayla said.

According to Yayla, this attack may prove to mark a change of policy for IS towards Turkey and to be a signal of more attacks to come.

"IS has established several cells inside Turkey and the Turkish counter-terrorism apparatus has been crippled by the post-coup purges in the security forces," he said. "Unfortunately this will be a very bloody fight."

State of emergency

Turkey has been under a state of emergency following an attempted coup in July. Turkish parliament approved a three-month extension to the current state of emergency on Tuesday, which will now be in effect until April 19. Deputy Prime Minister Numan Kurtulmus said the extension was necessary following the recent terrorist attacks across the country.