Nightclub terror attack in Istanbul: What we know

"(He) rained bullets in a very cruel and merciless way on innocent people who were there to celebrate New Year's and have fun," Sahin said.

Here's what we know:

Authorities have categorized the shooting as a terrorist attack

"We are aware that these attacks, carried out by various terror organizations against our country, are not independent from incidents happening in our region," Turkey President Recep Tayyip Erdogan said Sunday in a statement. "We are determined to eradicate threats and attacks against our country at their source."

He characterized the attack as an attempt at destabilizing the country through a chaotic attack on civilians.

"Turkey, continuing its fight against terror, is extremely determined to do whatever it takes in the region … to ensure the security and peace of its citizens," he said.

ISIS has claimed responsibility

The Islamic State group claimed responsibility for the attack, The Associated Press reported early Monday. A report from the Aamaq News Agency, which is linked to the terror group, said the attacker targeted the nightclub as "revenge for God's religion."

Turkish Deputy Prime Minister Numan Kurtulmus said the attack was in response to the country's "successful and determined" military operation against Islamic State fighters in norther Syria, according to The Associated Press.

"Wherever they may hide in 2017, we will enter their lair... With the will of God, with the support of our people, with all our national capacity, we will bring them to their knees and give them all the necessary response," he said.

No suspect has been identified

A manhunt was launched after the shooting, although Interior Minister Suleyman Soylu said no suspect was immediately identified. Kurtulmus told The Associated Press on Monday that authorities have gotten "fingerprints and a basic description" of the attacker.

"This was a massacre, a truly inhuman savagery," Soylu said, according to BBC News. "A manhunt for the terrorist is underway. Police have launched operations. We hope the attacker will be captured soon."

The Associated Press reported Monday that Turkish police have detained eight people in connection with the attack, but the gunman was not one of them.

Surveillance video showed the attacker wearing a Santa Claus hat

Reports initially claimed that the gunman wore a Santa Claus outfit during the attack on the Reina, although a review of surveillance footage revealed that the gunman had on only a Santa hat, according to The Associated Press.

The assailant was seen carrying a backpack and dressed in black as he shot down a police officer outside the nightclub. Inside the club, he wore a different outfit that included the hat, the AP reported.

Turkish Prime Minister Binali Yildirim denied that the gunman donned a Santa Claus outfit.

"There is no truth to this," he told reporters. "He is an armed terrorist as we know it."

Officials are investigating how the attacker escaped

Despite the quick reaction from authorities responding to reports of the nightclub shooting – a police station sits across the street from the Reina -- the gunman managed to slip past officers. Officials are working to determine how.

"There are over 300 security cameras in the club and the surrounding area, so investigators are coming through security (camera) footage," The Guardian reported. "One theory, believed by some who have seen some of the footage, is that the attacker, who left his gun at the scene, may have pretended to be an injured civilian in the immediate aftermath and chaos."

Turkish authorities say people from "many different nationalities" killed, wounded

Among the foreigners killed in the attack were three people from Lebanon, three Jordanians, one French citizen, seven Saudis and four Iraqis, The Associated Press reported, citing local reports and government officials.

In a statement, French President Francois Hollande vowed that the country "will pitilessly pursue the fight against this scourge with its allies."