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A terrorist remained on the loose Sunday after opening fire at a crowded nightclub in Istanbul during New Year's celebrations, killing at least 39 people.

The suspect killed a police officer and a civilian outside the Reina Club before entering and firing on revelers inside at about 1:30 a.m. (5:30 p.m. ET Saturday).

"Efforts to find the terrorist are continuing," Turkish Interior Minister Suleyman Soylu told reporters. "God willing, he will be caught in a short period of time. This was a massacre, a truly inhuman savagery."

Related: Scenes of Chaos and Mourning After Istanbul Attack

Twenty-eight of the dead were foreigners, Turkish Health Ministry officials said. Canadian, Iraqi, Saudi, Indian, Lebanese, Tunisian, Kuwaiti and Syrian citizens are among the dead.

An Arab Israeli woman was also killed, Israel's Foreign Ministry said, and her friends were wounded. Turkish Health Minister Recep Akdag said 65 people were wounded, four of them critically.

Among the wounded was one American citizen — Jake Raak, 35, of Greenville, Delaware, who was in Istanbul to celebrate New Year’s, his brother, Mike Raak, told NBC News.

Mike learned the news after receiving a call from his brother, who was hospitalized.

“I was in a club and I got shot,” Mike recalled Jake saying.

The injuries weren’t serious — the bullet struck Jake’s hip and traveled to his knee — and doctors expected him to be discharged and on his way home to the United States within the next day, Mike said.

Still, Mike added, his brother sounded horrified.

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“He was completely shaken,” he said.

As the manhunt unfolded, the U.S. Consulate in Istanbul warned Americans in the city to "shelter in place," and it tried to tamp down rumors circulating on social media that the U.S. government had forewarning of the attack. In a statement, the consulate denied that it had warned Americans to stay away from the area.

A handful of WNBA players, including Essence Carson, Chelsea Gray and Jantel Lavender of the Los Angeles Sparks, were next door when the attack took place. Sparks coach Brian Agler told The Associated Press that Carson texted him to say the three players were OK.

About two dozen WNBA players are in Turkey during their off-season playing in a league there. The WNBA told the AP that all of the players had been accounted for.

We're okay. Goodnight. — Essence T. Carson (@Pr3pE) January 1, 2017

Police guarded the scene of the attack Sunday in the Ortakoy district, close to the Bosphorus strait.

Istanbul provincial Gov. Vasip Sahin said the attacker was armed with a long-barreled weapon and "rained bullets in a very cruel and merciless way on innocent people who were there to celebrate New Year's and have fun," according to the Anadolu news agency.

While it had been reported that the assailant was dressed in a "Santa Claus" outfit, several sources, including Prime Minister Binali Yildrim, said that was incorrect.

Sinem Uyanik was in the club with her husband, who she told NBC News was shot three times but is expected to survive. She said she fainted briefly after two people were shot in front.

Turkish President Recep Tayyip Erdogan "vehemently" condemned the attack in a statement.

"Turkey continues its combat against terror and is absolutely determined to do whatever is necessary in the region to ensure its citizens safety and peace," he said.

National Security Council spokesman Ned Price reaffirmed U.S. support for Turkey, a NATO ally.

"That such an atrocity could be perpetrated upon innocent revelers, many of whom were celebrating New Year's Eve, underscores the savagery of the attackers," he said in a statement.

Security measures had been heightened in major Turkish cities, with police barring traffic leading up to key squares in Istanbul and the capital Ankara. In Istanbul, 17,000 police officers were put on duty, Anadolu reported.

Security operations still in progress. U.S. Citizens should shelter in place, limit movements & contact family & friends if you are fine. — ABDIstanbul (@ABDIstanbul) January 1, 2017

Ankara and Istanbul were targeted by several attacks in 2016 by ISIS or Kurdish rebels, which killed more than 180 people.

The attack in Istanbul prompted the New York Police Department to deploy some counterterrorism teams and patrols to nightspots, police officials said, although they cautioned that there were no specific or credible threats to the city.

Among the wounded was one American citizen — Jake Raak, 35, of Greenville, Delaware, who was in Istanbul to celebrate New Year’s, his brother, Mike Raak, told NBC News.

Mike learned the news after receiving a call from his brother, who was hospitalized.

“I was in a club and I got shot,” Mike recalled Jake saying.

The injuries weren’t serious — the bullet struck Jake’s hip and traveled to his knee — and doctors expected him to be discharged and on his way home to the U.S. within the next day, Mike said.

Still, Mike added, his brother sounded horrified.

“He was completely shaken,” he said.