British Aid Worker David Haines Apparently Beheaded by Islamic State Militants
09/13/2014 AT 06:50 PM EDT
The video emerged hours after the family of Haines issued a public plea on Saturday urging his captors to contact them.
Islamic State militants have beheaded two American journalists – Steven Sotloff and James Foley – and posted video evidence online. At the end of the last video showing the beheading of Sotloff, they threatened to kill Haines next and briefly showed him on camera.
The 44-year-old Haines was abducted in Syria in 2013 while working for an international aid agency.
The British government had managed to keep his kidnapping secret out of concern for his safety until the most recent video identified him as a captive.
British Prime Minister David Cameron late Saturday condemned his slaying as "an act of pure evil."
Cameron confirmed Haines's death in a statement after the British Foreign Office had said earlier that it was "working urgently to verify the video."
"This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil," Cameron said, adding that "my heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal."
"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," Cameron said.
On Sunday, Haines's brother, Mike Haines, issued a statement on behalf of their family that praised David for his work overseas.
"David was most alive and enthusiastic in his humanitarian roles," he said. "His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair."
Mike also painted David as a family-oriented regular "bloke" who leaves behind his second wife, Dragana, their 4-year-old daughter and an older daughter from a previous marriage.
"David was a good brother, there when I needed him and absent when I didn't," he said. "I hope that he felt the same way about me."
"He was, in the right mood, the life and soul of the party, and on other times the most stubborn, irritating pain in the ass," he added. "He would probably say the same about me."
With reporting by PHILIP BOUCHER