Islamic State militants released a video Saturday showing the beheading of aid worker David Haines, the latest in a disturbing series of such executions.
The 44-year-old Haines, who was British, was abducted in Syria in 2013 while working for an international aid agency. His whereabouts had been unknown.
The British government had kept the kidnapping secret out of concern for his safety until another video was recently released by Islamic State, also known as ISIS or ISIL.
ISIS’ video identified him as a captive, showing him in an orange jumpsuit and calling out Prime Minister David Cameron personally for his soon-to-be beheading.
The British Foreign Office had said late Saturday that it was "working urgently to verify the video," but Cameron’s statement left little room for ambiguity:
"The murder of David Haines is an act of pure evil. My heart goes out to his family who have shown extraordinary courage and fortitude."
"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," Cameron added.
President Barack Obama commented that "The United States strongly condemns the barbaric murder of UK citizen David Haines by the terrorist group ISIL."
"Our hearts go out to the family of Mr. Haines and to the people of the United Kingdom. The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve."
"We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice."
As he said in his address to the nation last week, Obama vowed "degrade and destroy this threat to the people of our countries, the region and the world."
As in the sickening beheading videos of Steven Sotloff and James Foley, the executioner, dressed in black and speaking with an English accent, addressed Cameron.
"This British man has to the pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State," the militant says.
"If you, Cameron, persist in fighting the Islamic State then you like your master Obama will have the blood of your people on your hands."