Istanbul attack
Flowers and a Turkish flag are placed near the entrance of Reina nightclub, which was attacked by a gunman, in Istanbul, Turkey, Jan. 2, 2017. REUTERS/Yagiz Karahan

The attacker involved in the deadly shooting at a popular nightclub in Istanbul early Sunday is a highly professional assassin, security experts claimed Tuesday. The gunman — who killed 39 people and injured nearly 70 others — fled the scene after firing at over 600 people, and is still at large.

The way the attacker carried out the attack shows that "he is absolutely a killer and he probably shot at humans before," English-language Daily News quoted anti-terror expert Abdullah Agar as saying. "The attacker is determined, faithful, practical, coldblooded expert and knows how to get results ... he probably fired bullets in real clash zones."

The attacker, who is believed to be in his 20s stormed the Reina club after killing a policeman and a civilian. The Islamic State group on Monday claimed responsibility for the attack, saying that a "soldier of the caliphate" had carried out the shooting in response to Turkish military operations against ISIS in northern Syria.

While a massive manhunt is underway for the gunman, who has not been identified, Turkish media on Tuesday ran a "selfie video" of a man they say is the attacker.

The clip, which was later released by the Associated Press, shows the alleged gunman filming himself with a cellphone at Istanbul's Taksim square. It is unclear if the video was filmed before or after the New Year's massacre, the AP reported.

On Tuesday, Turkey's state-run news agency Anadolu said that 14 people have so far been detained and are being questioned in connection with the attack. It reportedly said police were receiving numerous reports of sightings and tips from citizens, following the release of photos and videos of the alleged gunman.

A Turkish police handout picture made available of a suspect in Istanbul nightclub attack which killed at least 39 people on New Year's Eve. Photo released on Jan. 2, 2017. REUTERS/Reuters TV/Handout

So far, there have been several conflicting reports about the identity of the gunman. On Monday, local media reported that the attacker was believed to be from a Central Asian nation and may have been part of the same cell that targeted Istanbul's Ataturk Airport in June 2016. On Tuesday, Haber Turk newspaper reported, citing sources, that the attacker is believed to be a member of China's Muslim Uighur minority.

Turkey's crucial tourism industry has suffered a great deal over the recent attacks in the country.

On Tuesday, Turkey's prime minister slammed the Obama administration for backing Syrian Kurdish forces, which Turkey considers to be terrorists. Binali Yildirim also urged President-elect Donald Trump to put an end to "this shame."