An Israeli spokesman revealed Wednesday that slain American journalist Steven Sotloff was also a citizen of Israel.

Sotloff was kidnapped in Syria in August 2013, and ended up in the hands of the Islamic State (IS). A video released Tuesday showed Sotloff being executed by his captors.

A former hostage who was held with Sotloff reported that the journalist would pretend to be sick so that he could fast during the Jewish holiday Yom Kippur, according to Israeli newspaper Yedioth Ahronoth.

Sotloff's work also appeared in Israeli news publications, a fact that the outlets withheld out of caution.

"We refused to acknowledge any relationship with him in case it was dangerous for him," Jerusalem Report editor Avi Hoffman told Reuters.

Israeli media reported that Sotloff immigrated to Israel in 2005, became a citizen, and attended the Interdisciplinary Center Herzliya, a college near Tel Aviv. The 30-year-old also studied journalism at the University of Central Florida, and wrote for the Christian Science Monitor, Time Magazine, the World Affairs Journal, and other outlets.

The White House confirmed Wednesday morning that the video showing the freelance reporter's beheading was authentic.

"Overnight, our government confirmed that, tragically, Steven was taken from us in a horrific act of violence," President Obama said in a news conference.

"Those who make the mistake of harming Americans will learn that we will not forget and that our reach is long and that justice will be served."

The death video comes two weeks to the day that a video showing the execution of American photojournalist James Foley was released by IS. The 40-year-old was covering the Syrian civil war when he was kidnapped nearly two years ago.

Foley's death was prompted by U.S. airstrikes in Iraq and Syria, and the missile attacks have continued in the weeks since. The IS said on Tuesday's video that Sotloff's death is in retaliation for the continued military action.

"[As long as missiles] continue to strike our people, our knife will continue to strike the necks of your people," a masked man stated.

It is unclear how many hostages IS has in their custody, although a U.S. official revealed there are a "small number" of Americans being held captive. The life of a British citizen, David Cawthorne Haines, was also threatened on video.