Update at 7:05 p.m. EDT
The militant group that calls itself the Islamic State has released a video that purportedly shows the beheading of British aid worker David Haines.
The authenticity of the video, which appeared online Saturday, has not been confirmed.
The organization, also known as ISIS, had threatened to kill Haines just under two weeks ago, in an earlier video that showed the beheading of an American journalist. Haines' family issued a public plea to his captors asking ISIS to make contact with them, as The Associated Press reported earlier in the day.
Haines, an international aid worker, was abducted in Syria in 2013. "The British government had managed to keep his kidnapping secret out of concern for his safety until the most recent video Islamic State video identified him as a captive," the AP writes.
Reuters reports that Haines, a father of two from Perth, Scotland, was working for the French agency ACTED when he was kidnapped.
ISIS previously released two videos showing the beheadings of two American journalists, which were confirmed by U.S. officials to be authentic. The first video, which showed the killing of James Foley, was released on August 20. The second, which showed the beheading of Steven Sotloff and contained the threat directed towards Haines, was released on Sept. 2.
The newest video, like those previous videos, appears to name another Western hostage as a future target.
Alice Fordham, reporting for NPR's Newscast unit, says the video begins with footage of British prime minister David Cameron. The two previous videos began with footage of President Obama.
The man identified as Haines, Alice reports, is shown "kneeling in a featureless desert in an orange robe."
The video appears recent, Alice says: "The assailant refers to the bombing of Iraq's Haditha dam a week ago. Another man identified as a British hostage appears at the end as the masked man exhorts Prime Minister Cameron to stop fighting the Islamic State."
Cameron has tweeted a response, writing, "We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."
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