Last Updated Aug 19, 2014 11:00 PM EDT
The Islamic extremist group that has overrun parts of Syria and Iraq released a video Tuesday that apparently shows the beheading of American journalist James Foley. The militant group also threatened to kill another U.S. journalist if the U.S. does not halt its airstrikes in Iraq.
The Islamic State of Iraq and Syria, known as ISIS, released a video titled "A Message to America" that appeared on several social media websites. YouTube removed it shortly after it was posted.
The video shows a militant using a knife to decapitate a man the group identifies as James Foley. Foley, a 40-year-old freelance journalist who had been based in Boston, disappeared in northwest Syria on Thanksgiving Day, Nov. 22, 2012.
The Obama administration said it had not determined whether the video is authentic, but two U.S. officials told The Associated Press they believe the person executed in the video was Foley. Foley's family in Rochester, New Hampshire, confirmed the death in a Facebook posting.
"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," his mother, Diane Foley, said in the post. "We thank Jim for all the joy he gave us. He was an extraordinary son, brother, journalist and person."
At the end of the video, a second hostage is seen. He is identified in the video as freelance photojournalist Steven Joel Sotloff. An ISIS extremist says Sotloff will be the next to die if the United States does not stop its attacks on ISIS positions in northern Iraq. CBS News has not confirmed the identity of the second man.
National Security Council spokeswoman Caitlin Hayden said the administration was working to determine whether the video was genuine.
"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," she said in a statement.
President Barack Obama was briefed on the ISIS video and will continue to receive regular updates on the matter, deputy White House press secretary Eric Schultz said.
Foley was on assignment for Agence France-Presse and the Boston-based media company GlobalPost when he disappeared in Syria in 2012.
Philip Balboni, GlobalPost CEO and co-founder, said the company was waiting for the administration's evaluation of the video, but said Foley's parents were grateful for the public's support and prayers.
"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," Balboni said in a statement. "We ask for your prayers for Jim and his family."
The oldest of five children, Foley grew up in New Hampshire and studied history at Marquette University. He was a teacher before switching careers to journalism. He had worked as a journalist in the Middle East for five years when he was kidnapped in Syria.
Foley had been take captive once before. In 2011, he was taken prisoner by forces loyal to Libyan dictator Moammar Gadhafi and held for 44 days before being released.
In an interview after his release, Foley told CBS News' "The Early Show" that he had gone with some colleagues to report from the front lines of the fighting.
"It's very important in this kind of war to see if what the rebels are saying they're doing is actually true," he said.
Foley told "The Early Show" their captors accused the journalists of being spies and asked them the same questions over and over again until they were finally set free.
Sotloff, said to be the man whose life ISIS threatens at the end of the video, disappeared in July 2013 while reporting in northern Syria. His work has appeared in Time and Foreign Policy magazines. The freelance journalist had reported from Libya and Yemen in addition to Syria.
The New York-based Committee to Protect Journalists says about 20 journalists are missing in Syria.
"The barbaric murder of journalist James Foley, kidnapped in Syria and held almost two years, sickens all decent people. Foley went to Syria to show the plight of the Syrian people, to bear witness to their fight, and in so doing to fight for press freedom," CPJ Chairwoman Sandra Mims Rowe said in a statement.
Among the missing journalists is American Austin Tice. His family says the freelance journalist was taken captive near Damascus, Syria, on Aug. 14, 2012.
In 2002, American journalist Daniel Pearl was kidnapped by militants in Pakistan and later beheaded by al Qaeda member Khalid Sheikh Mohammed, who is now in U.S. custody.
In her Facebook post, Diane Foley pleaded with extremists to spare their captives' lives.
"We implore the kidnappers to spare the lives of the remaining hostages," she said. "Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world."