British aid worker David Haines has been executed by ISIS militants, according to a video posted Saturday to a website associated with the group, making him the third Western captive to be killed by the Islamist extremist group in recent weeks.
The ISIS video post showing Haines' beheading called his execution "a message to the allies of America."
It is produced very similarly to the videos that showed the executions of American journalists James Foley and Steven Sotloff, the last of which included Haines and the threat that he'd be killed next.
The new video pictures a masked ISIS militant placing his hand on another captive, whom he identified as Alan Henning, a British citizen.
In a tweet, British Prime Minister David Cameron called "the murder of David Haines" an "act of pure evil."
Cameron added, "We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."
Haines offers brief scripted comments on the video, as does the man who kills him.
Directing his remarks at Britain, the executioner -- who sounds like the man who killed Foley and Sotloff -- says, "Your evil alliance with America, which continues to strike the Muslims of Iraq and most recently bombed the Haditha dam, will only accelerate your destruction and claim the role of the obedient lap dog.
"Cameron will only drag you and your people into another bloody and unwinnable war."
Retired Lt. Col. Rick Francona -- an Air Force veteran intelligence officer and CNN military analyst -- surmised that if ISIS planned to dissuade Britain for teaming up with the United States, the group will be disappointed.
"ISIS has just guaranteed British cooperation with the Americans on all phases of what we're going to be doing," Francona said. "... I think this is now a Western fight; it's not just a U.S. fight."
ISIS execution videos strikingly similar
Brother: Haines 'just another bloke' who lived to help others
News of the gruesome killing came the same day that the 44-year-old Haines' family released a brief message to his captors through the British Foreign Office.
In it, the family says, "We have sent messages to you to which we have not received a reply. We are asking those holding David to make contact with us."
Haines' face became known to the world in the ISIS video, released September 2, in which he looks forward and kneels as a masked ISIS militant stands behind him.
The militant says in that video, "We take this opportunity to warn those governments who've entered this evil alliance of America against the Islamic State to back off and leave our people alone."
British officials said after the video's release that they had sent troops to try to rescue an unidentified British citizen "some time ago," but failed. They released no other details.
Mike Haines, in a statement early Sunday, through the British Foreign Office, noted that his brother leaves behind two children and his wife Dragana. He described his brother as "just another bloke" whose "childhood was centered around our family" and who was "brought up to know right from wrong."
David Haines worked for the Royal Mail, then joined the Royal Air Force. He later worked with the United Nations in the Balkans, where "he helped whoever needed help, regardless of race, creed or religion," according to his brother.
"During this time, David began to decide that humanitarian work was the field he wanted to work in," Mike Haines said. "... David was most alive and enthusiastic in his humanitarian roles."
After working for ScotRail, David Haines went on to get a job as a logistics and security manager for the Paris-based humanitarian Agency for Technical Cooperation and Development.
He was abducted in March 2013 near a refugee camp in Atmeh, Syria, where he was working to arrange for the delivery of humanitarian aid to people staying at the camp. He had previously worked on aid operations for victims of conflict in the Balkans, African and other parts of the Middle East, according to an ACTED spokesman.
"His joy and anticipation for the work he (did) in Syria is, for myself and family, the most important element of this whole sad affair," Mike Haines said. "He was and is loved by all his family and will be missed terribly."