Istanbul (CNN) ISIS claimed responsibility for the New Year's attack at Istanbul's Reina nightclub that left 39 people dead, but authorities are still scrambling to find the killer.

ISIS' claim, made in a statement posted to Twitter, cannot be independently verified by CNN. But it boasted about the first major terrorist attack of 2017.

"In continuation of the blessed operations which ISIS carries out against Turkey, a soldier of the brave caliphate attacked one of the most popular nightclubs while Christians were celebrating their holiday," the statement read.

Both Turkish and US officials have called the attack an act of terrorism.

Here's the latest on the investigation, the victims and why Turkey keeps getting targeted:

The investigation

Investigators have found the fingerprints of the suspect and know what he looks like, Deputy Prime Minister Numan Kurtulmus said Monday.

Turkish state-run media say police provided this photo of the suspect in the Istanbul nightclub attack. CNN cannot confirm when or where the photo was taken.

Eight people have been detained in connection with the attack, Kurtulmus said, but the suspect was not among them.

"We will find him, no delay," Turkish Prime Minister Binali Yildirim said.

Footage of the attacker showed him shooting a security guard and police officer at the entrance of the nightclub. Turkish authorities said they believe he carried out the attack alone.

Earlier Monday, the militant Kurdistan Worker's Party, or PKK , distanced itself from the attack.

"No Kurdish forces have anything to do with this attack," the PKK said. "The Kurdish freedom fight is also the fight for democratization of Turkey. That's why we won't target innocent and civilian people."

The victims

Those killed in the attack were from 14 countries , including India, Morocco, Jordan, Canada, Russia, Tunisia, Saudi Arabia and Kuwait.

Istanbul attack victims Turkey - 11

Saudi Arabia - 7

Iraq - 3

Lebanon - 3

India - 2

Jordan - 2

Morocco - 2

Tunisia - 2 (one French dual national)

Belgium - 1 (Turkish dual national)

Russia - 1

Canada - 1

Kuwait - 1

Syria - 1

Israel - 1 Source: Foreign ministries/news reports

Security guard Fatih Cakmak was one of the first victims killed. He had considered himself lucky after surviving a December attack outside Istanbul's Vodafone Arena , where he was also working security.

"He went to work, and now he's gone," said his father, Hassan Cakmak, who said he can't believe he survived his 32-year-old son.

Lubna Ghaznawi, a young female entrepreneur from Saudi Arabia, was also killed. The 34-year-old worked full-time as a communications manager for one of Saudi Arabia's largest banks and also co-owned a start-up with her sister called Exclusave Card, the first student discount card in Saudi Arabia.

"She was an optimist and loved going to new places," her friend Seham al-Shahrani told CNN. "Laughter and happiness filled whatever place she went to."

Dozens of people were hospitalized. As of Monday, 46 were still being treated, including one American, according to the Istanbul governor's office. A handful of the injured were in critical condition.

'We were having fun'

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Witnesses described how the festive evening turned into a bloodbath.

"We were having fun. At first we thought it was a fight, then there was a lot of gunfire," witness Yunus Turk told CNN

"After the gunfire everyone started to run toward the terrace. We ran as well. There was someone next to me who was shot and fell on the floor. We ran away and hid under the sofas."

Another witness said he didn't know how many attackers there were, but he saw one person and hid.

"I got shot in the f****** leg, man," he told journalists as he was taken into an ambulance. "These crazy people came in shooting everything."

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On Sunday, the club issued a statement on its Facebook page

"This terrible incident is a terror attack against our citizens' peace, brotherhood, serenity, economy, tourism and against our nation," the statement read.

"Our hearts bleed and the bullets are in our heart."

Why Turkey keeps getting targeted

Turkey's deputy prime minister said he believes the attack was committed in response to Operation Euphrates Shield , Turkey's ground operation against ISIS in Syria.

But the country is "fighting a two-front war -- against the Kurdish Workers' Party, and against the Islamic State," CNN intelligence and security analyst Bob Baer said.

Turkey shares a 500-mile (800-kilometer) border with Syria, making it relatively easy for ISIS fighters and supplies to cross.

"That border is so porous -- weapons, explosives, people coming across it," Baer said.

Both ISIS and Kurdish militants have launched attacks in Turkey, which is also reeling from a bloody and failed military coup in July.

Photos: Attack at Istanbul nightclub Photos: Attack at Istanbul nightclub This still photo, taken from surveillance footage and released on Monday, January 2, is believed to show the gunman responsible for carrying out a New Year's Day attack on the Reina nightclub in Istanbul. The popular nightclub was attacked shortly after midnight on Sunday, January 1. At least 39 people were killed and 69 were wounded, Turkey's Interior Minister said. Authorities are still searching for the attacker. Hide Caption 1 of 19 Photos: Attack at Istanbul nightclub A Turkish special forces officer stands near the Reina nightclub on January 2. ISIS claimed responsibility for the attack in a statement posted to Twitter. Hide Caption 2 of 19 Photos: Attack at Istanbul nightclub A friend of someone killed in the attack reacts near victims' pictures outside the nightclub on January 2. Hide Caption 3 of 19 Photos: Attack at Istanbul nightclub A police officer inspects cars near the scene on January 2. Hide Caption 4 of 19 Photos: Attack at Istanbul nightclub People carry the coffin of Yunus Gormek, a victim of the attack, during a funeral ceremony in Istanbul on January 2. Hide Caption 5 of 19 Photos: Attack at Istanbul nightclub People mourn outside the Forensic Medical Center in Istanbul on January 1. Hide Caption 6 of 19 Photos: Attack at Istanbul nightclub Relatives of Ayhan Arik, one of the victims of the attack, cry during a funeral ceremony in Istanbul on January 1. Hide Caption 7 of 19 Photos: Attack at Istanbul nightclub An ambulance rushes from the scene of the attack on January 1. Hide Caption 8 of 19 Photos: Attack at Istanbul nightclub A woman is consoled at the site of the attack. Hide Caption 9 of 19 Photos: Attack at Istanbul nightclub Medics wheel a stretcher at the scene. Hide Caption 10 of 19 Photos: Attack at Istanbul nightclub Police officers stand guard. Hide Caption 11 of 19 Photos: Attack at Istanbul nightclub People leave the scene of the attack. Hide Caption 12 of 19 Photos: Attack at Istanbul nightclub A medic reacts near the scene of the attack. Hide Caption 13 of 19 Photos: Attack at Istanbul nightclub Hide Caption 14 of 19 Photos: Attack at Istanbul nightclub A wounded victim is rushed from the scene. Hide Caption 15 of 19 Photos: Attack at Istanbul nightclub Medics and security officials work at the scene of the attack. Hide Caption 16 of 19 Photos: Attack at Istanbul nightclub A wounded person is put into an ambulance. Hide Caption 17 of 19 Photos: Attack at Istanbul nightclub People walk in the rain near the scene of the attack. Hide Caption 18 of 19 Photos: Attack at Istanbul nightclub An ambulance transports those wounded in the attack. Hide Caption 19 of 19

ISIS has typically refrained from claiming responsibility for attacks in Turkey to create "an environment of suspicion in Turkish politics," analyst Soner Cagaptay wrote for CNN last year.

So ISIS' claim of responsibility for Sunday's nightclub attack could mark a turning point.

"I think with Istanbul, it's a declaration of war on Turkey," Baer said.

Meanwhile, Turkish security forces clash almost daily with rebel PKK militants, mostly in predominantly Kurdish parts of southeastern Turkey.

In December, a pair of bombings in Istanbul killed 44 people and wounded 155 others in an attack by a breakaway group of the PKK. The two explosions occurred after a heavily attended soccer game at Besiktas Vodafone Arena.

Also in December, a car bomb exploded near a public bus, killing 13 soldiers in the central province of Kayseri. Three days later, a gunman assassinated Russia's ambassador to Turkey at an Ankara art gallery.

'The issue of a copycat'

So how could the attack at the Reina nightclub have implications elsewhere?

Since the assailant used a gun, and apparently started by killing the security guard, a similar attack could happen anywhere, CNN senior law enforcement analyst Tom Fuentes said.

"You have the issue of a copycat," he said. "So here in the United States, we have over 300 million guns at large in our population. So we're talking about one person getting a hold of one of those guns and launching an attack and doing it on his own. ... So it's almost impossible to stop, if that's what we end up with -- a lone wolf who wants to get a gun."