ISIS Claims Istanbul Nightclub Attack That Killed 39 as Manhunt Continues

One American was injured in the attack on a popular Istanbul nightclub.

— -- ISIS claimed responsibility for the shooting rampage at a New Year's Eve celebration at an Istanbul nightclub that killed at least 39 people and injured nearly 70 more, including one American, as authorities continued a manhunt for the assailant.

The gunman entered the popular Reina club early Sunday morning armed with what authorities described as a long-barreled weapon. Surveillance video footage of the moments leading up to shooting spree shows the shooter firing his weapon as he approached the club. He killed a policeman and a civilian outside of the club before "[raining] bullets in a very cruel and merciless way on innocent people" inside the club, according to Visip Sahin, the governor of Istanbul.

Most of the victims in the massacre were shot at close range or took bullets directly to the head, an official morgue report said.

"One of the hero soldiers of the caliphate hit one of the most famous nightclubs where Christians celebrate their apostate holiday ... so the Turkish government knows that the Muslim blood it is spilling through its airstrikes and artillery will strike back in the midst of its nation," the ISIS statement said.

The Reina club had attracted a diverse crowd of between 400 and 500 patrons from foreign countries as well as Turkey to ring in the new year.

Among the nearly 70 people injured was a small business owner from Greenville, Delaware, Jake Raak, who was shot in the leg, family members told ABC station WPVI-TV in Philadelphia. Raak's brother, Michael, told WPVI-TV the bullet may have struck his cellphone, which prevented him from being injured more seriously.

At a press conference Sunday, authorities said the assailant arrived wearing a jacket and pants, and is believed to have left wearing different clothing.

ABC News' Darren Reynolds, Devin Villacis, Engin Bas, Benjamin Gittleson, Troy McMullen and The Associated Press contributed to this report.