ISIS has tried to send a warning to the U.S. in about the most horrifying manner imaginable — by broadcasting the execution of an American citizen captured in Syria more than 600 days ago.

The video purports to show the execution of photojournalist James Foley.

The four-minute video opens with a clip of President Barack Obama authorizing U.S. airstrikes in Iraq. After this intro, a title screen appears with the words "a message to America," followed by an American citizen they identify as James Foley saying that U.S. leaders are his "real killers." Foley is apparently executed moments later by a man in a black robe — an apparent native English speaker who talks with a British accent — holding a knife.

A photo of Foley in Aleppo, Syria, August 2012. Nicole Tung/FreeJamesFoley.org Caitlin Hayden, a representative for the White House's National Security Council, said in a statement that the U.S. intelligence community was still working to confirm the authenticity of the video.

"We have seen a video that purports to be the murder of U.S. citizen James Foley by ISIL," Hayden said. "The intelligence community is working as quickly as possible to determine its authenticity.If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends. We will provide more information when it is available."

The White House said late Tuesday that Obama had been "briefed" on the video by Deputy National Security Advisor Ben Rhodes and would continue to receive regular updates throughout the night.

On the Facebook page "Free James Foley," his mother Diane released the following statement:

We have never been prouder of our son Jim. He gave his life trying to expose the world to the suffering of the Syrian people.

We implore the kidnappers to spare the lives of the remaining hostages. Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world.

We thank Jim for all the joy he gave us. He was an extraordinary son, brother, journalist and person. Please respect our privacy in the days ahead as we mourn and cherish Jim.

Foley was a GlobalPost contributor, and the online publication spent "many, many, many hundreds of thousands of dollars" to find him during his first six months in captivity, according to a report by Boston public radio station WBUR. At that point, his family believed that Foley, who was captured in northwest Syria on Thanksgiving Day 2012, was in Syrian military detention, and being held as a prisoner of Bashar Assad's regime.

On May 3, 2013, Global Post reported that it "believe[d] the Syrian government is holding [Foley] in a detention center near Damascus," a determined reached after "a five-month investigation inside Syria and the wider Middle East."

James Foley in Tripoli, Libya in August 2011. Jonathan Pedneault

Although ISIS is an offshoot of Al Qaeda and the successor organization of Al Qaeda in Iraq, the group did not officially form until April 2013, months after Foley was captured (Jabhat al Nusra, Syria's other major Jihadist group, wasn't designated a terrorist organization by the U.S. until a month after Foley's abduction). If the man in the video was Foley it remains to be seen how and whether he was transferred from possible regime custody to ISIS.

Foley had reported from a number of other conflict zones, including Afghanistan and Libya, where he was held captive by the Gaddafi regime for 44 days. Foley later published a series of reports on GlobalPost detailing his over one-month in various Libyan prisons.

James Foley in Syria, 2012. Nicole Tung

American journalist Clare Morgana Gillis was held captive with Foley in Tripoli. She shares this story of their time together with Syria Deeply:

We shared a cell for two and a half weeks, and every day he came up with lists for us to talk through. Top 10 movies. Favorite books. The fall of the Roman Empire and the rebirth of Western civilization. Which famous person would you most like to meet? What’s your life story? How does war change you? How can we be better people when we get out of here?

When I was in tears after a six-hour interrogation that ended at sunrise, he observed matter-of-factly, “It’s their job to break you. They did it to you today, and they’ll do it to me tomorrow. Get some sleep.”

GlobalPost, for whom Foley reported from Syria, stressed that the video's authenticity had not yet been verified. Philip Balboni, GlobalPost's CEO, released the following statement, "On behalf of John and Diane Foley, and also GlobalPost, we deeply appreciate all of the messages of sympathy and support that have poured in since the news of Jim’s possible execution first broke. We have been informed that the FBI is in the process of evaluating the video posted by the Islamic State to determine if it is authentic."

However, Stars and Stripes, the U.S. military affairs-focused newspaper for which Foley reported from Afghanistan, is reporting that Foley had been killed.

Paul Szoldra contributed to this report.