Update at 8:50 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 independently confirmed by NPR.
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. This weekend, Haines' family had issued a public plea to his captors through the British Foreign and Commonwealth Office. The family asked ISIS to make contact with them.
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.
The BBC reports that the 44-year-old father of two from Perth, Scotland, was kidnapped shortly after he began working with a French relief agency called ACTED. At his posting there, Haines was "working in the Atmeh refugee camp ... supplying water, food and tents."
Over the past decade and a half he'd worked with a variety of aid agencies, writes the BBC: "He had worked with a German charity on post-war reconstruction projects in Croatia, including housing and demining. He was also involved in efforts to help displaced people to return to their homes. In 2011 he became Head of Mission in Libya for Handicap International," an organization that works to help vulnerable people with disabilities.
"The following year he joined another agency, the Nonviolence Peaceforce (NP), and went to South Sudan," where he worked as an unarmed civilian peacekeeper.
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.
NPR's correspondent Alice Fordham, reporting for our 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 to be 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."
President Obama said in a statement that the U.S. "strongly condemns the barbaric murder of UK citizen David Haines by the terrorist group ISIL," and pledged to work with the U.K. and other nations to "bring the perpetrators of this outrageous act to justice."