ISTANBUL — Leanne Nasser was a bright-eyed Arab-Israeli teenager, in Istanbul with friends for the New Year, despite her father's concerns about safety. Fatih Cakmak, who survived a bomb attack only weeks ago, was hired to work security for a popular nightclub.

Both were among those who died early Sunday when a gunman, brandishing an assault rifle, stormed Istanbul's famed Reina nightclub on the banks of the Bosporus, gunning down unsuspecting New Year's revelers in a rampage that was one of the city's worst mass killings in recent memory.

The assailant remained at large late Sunday, and unidentified except for blurred glimpses of him in security camera footage that showed gunshots sparking off the pavement and victims crumpling to the ground.

Thirty-nine people were killed, many of them foreigners, in the latest in a string of assaults that have roiled Turkey as it battles insurgents at home and across the border in war-torn Syria. At least 70 people were wounded. There was no claim of responsibility for the attack.

A U.S. State Department official, who spoke only on condition of anonymity, said one American man was among those wounded.

Among those killed were an Iraqi student, a Turkish policeman and two Lebanese fitness trainers. Authorities were working Sunday to identify the dead, and citizens of at least eight countries, including Turkey, were killed in the assault.

“Please answer my comment, and tell me you have not died,” Facebook user Sheery Rudan posted on the profile of Mustafa Jalal, 22, an Iraqi student from Kirkuk. The Istanbul school where he studied, Kemerburgaz University, announced his death on Twitter.

Hassan Alaa, who was close to Jalal, struggled with the news of his boyhood friend's death. Jalal, who was an only child, was active, outgoing, and loved cars and swimming, he said.

“I can't believe this,” Alaa said.

The assault, which targeted a posh, sprawling venue popular with Istanbul's elite, recalled similar attacks on a concert hall in Paris in 2015 and nightclub in Orlando in 2016. And like the previous attacks — with their tally of young victims, all of whom had been enjoying a night out — the carnage left Istanbul reeling from a similar sense of shock and grief.

It began just after 1 a.m., when the assailant shot dead a 22-year-old police officer, Burak Yildiz, and a chauffeur for a tourism company, Ayhan Arik, on the street outside the club, according to Turkish media reports. Arik had taken foreign guests to the nightclub.

The sound of gunshots sent panicked patrons scrambling for cover as the gunman came inside, witnesses said.

“At first we thought some men were fighting with each other,” a Lebanese woman who gave her name as Hadeel told the Reuters news agency. She was in the club with her husband and a friend.

“We heard the guy screaming ‘Allahu Akbar,'?” she said. “We heard his footsteps crushing the broken glass. We got out through the kitchen, there was blood everywhere and bodies.”

Others did not survive the assault.

Mustafa Sezgin Seymen, 32, was at Reina with his fiancee, Sezen Arseven. She was wounded; Seymen was killed.

“I'm returning without you from the place we went together,” Arseven wrote of Seymen in a public Facebook post Sunday.

Officials on Sunday called the nightclub attack a massacre and an act of terrorism.

Turkish President Recep Tayyip Erdogan said the attack was meant to “trigger chaos.”

He vehemently condemned “the terror attack in Istanbul's Ortakoy neighborhood in the first hours of 2017” and offered condolences for those who lost their lives, including the “foreign guests.”

Sunday's incident was the fourth major attack in Turkey in less than a month, including the high-profile assassination of the Russian ambassador by an off-duty Turkish policeman, and a brazen car bomb assault against riot police at a soccer stadium in Istanbul. That attack, which killed 44 policemen, was claimed by separatist Kurdish militants, who have waged a decades-long insurgency against the Turkish state.

The attacks have posed a severe challenge to Erdogan's government.

Fatih Cakmak, the security guard, was working at the stadium when the car bomb detonated on Dec. 11, Turkish media reported.

“They killed my brother. He died for nothing,” Erhan Cakmak said outside the central morgue in Istanbul on Sunday, in a video circulated by the private Dogan news agency.

In Israel, the father of Leanne Nasser, speaking to local media, said he implored his 19-year-old daughter not to travel to Istanbul because of recent attacks.

United States officials, meanwhile, denied reports in Turkish new outlets and on social media that its security agencies knew in advance that the nightclub was at risk of a terror attack. The U.S. Embassy in Ankara said in a statement that “contrary to rumors circulating in social media, the U.S. Government had no information about threats to specific entertainment venues, including the Reina Club.”

Associated Press contributed.