U.S. intelligence analysts have determined that the video showing the execution of American photojournalist James Foley by Islamic State militants is real, National Security Council spokeswoman Caitlin Hayden said Wednesday.
"We have reached the judgment that this video is authentic," Hayden said.
President Obama spoke about Foley's death from his vacation in Massachusetts, saying the "entire world is appalled" and the nation "heartbroken" by the news of Foley's murder.
"We will continue to confront this hateful terrorism and replace it with a sense of hope," Obama vowed.
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In London, the United Kingdom's Foreign Secretary Philip Hammond said the beheading may have been undertaken by a British national.
In an interview with the BBC, Hammond acknowledged that the militant shown in the video with Foley — who was seized in Syria in 2012 while covering the civil war there — spoke with a British accent. It's possible that the perpetrator of the shocking killing could be British, he said.
"We're absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities, making jihad with IS and other extremists organizations," Hammond told the BBC.
"This is something we have been tracking and dealing with for many, many months and I don't think this video changes anything," he said. "It just heightens awareness of a situation which is very grave."
In the video, posted by the media arm of the Islamic State and titled "A Message to America," a man with a shaved head is seen kneeling beside a masked militant. The man, possibly Foley, 40, delivers what appears to be a scripted statement in which he asserts that U.S. airstrikes on Islamic State militants in Iraq "signed my death warrant."
The video concludes with another missing U.S. journalist, Steven Joel Sotloff of Miami, and tells President Obama his life "depends on your next decision."
"IS is clearly sending a message to the U.S. government with this act, but it's also sending a message to the American people," said Raffaello Pantucci, a researcher at Royal United Services Institute in London, a security think tank. "The American public will look at this and think: 'What is Obama doing (in Iraq)?'"
In a statement on a webpage created to rally support for Foley, his family confirmed his death. His mother said he "gave his life trying to expose the world to the suffering of the Syrian people."
Pantucci said that British government estimates suggest there may be as many as 500-880 British nationals involved in jihad activities for IS. "But we don't really know what that means. We don't know if that is nationals that have gone out there, stayed out there, died there, or have lived there all along," he said.
In 2002, Wall Street Journal reporter Daniel Pearl was abducted in Pakistan by British national Ahmed Omar Saeed Sheikh. Pearl was later killed in Pakistan, although the specific circumstances of his death remain unclear.
"Our hearts go out to the family of journalist James Foley. We know the horror they are going through." Pearl's mother, Ruth, said in a tweet from the Daniel Pearl Foundation.
British Prime Minister David Cameron's office said in a statement he was cutting short his vacation and returning to the capital to discuss the situation in Iraq and Syria and the threats posed by IS.
"The U.K. has never really been able to tackle the problem of radicalization," Pantucci said. "There's been a trend of individuals who have been drawn to these (Mideast) conflicts from here. The thing about Syria and Iraq is that it is such an accessible narrative. These people can go look online and just decide to participate. With it's proximity to Europe it's just so easy to do."
Christopher Davidson, a Mideast expert at Durham University, said there's been a lack of attention to the flow of Westerners to the region. "As long as they have been supposedly fighting the (Syrian President Bashar) Assad regime, authorities have turned a blind eye to it. Now that they are going to Iraq, we are starting to experience the blow back," he said.