UPDATED: Sept. 13, 6:40 p.m. EDT
On Saturday, ISIS released a video purporting to show Haines execution.
David Haines is the British hostage shown in the video depicting the apparent execution of US-based journalist Steven Sotloff by the Islamic State of Syria and Iraq, or ISIS.
In the video, Haines, 44, is shown wearing orange clothing and named as the next target for execution.
This is what we know about Haines:
He was captured in early 2013 from a refugee camp in northern Syria, close to the Turkish border.
Haines was working for the French non-governmental organisation ACTED when he was taken, along with an Italian named Federico Motka.
While other countries are willing to pay substantial ransoms for the safe return of their citizens, the British government has a strict policy of not doing such deals for fear of encouraging further hostage-taking. Motka was released earlier this year.
Haines, along with the executed US journalist James Foley and other hostages, was the subject of an attempted rescue by the US military earlier this year.
He went to school at Perth Academy in Scotland, has two daughters, and currently lives in Croatia.
Although born in Yorkshire, Haines grew up in Scotland. He currently lives with his Croatian wife Dragana, whom he married in 2010, according to her Facebook profile, and their 4-year-old daughter.
According to the Daily Mail, his elder daughter, who is 17 and from an earlier marriage, has written online posts saying: "I miss my dad. I would do anything to have him home."
He has military experience and spent years working in various war zones.
According to his LinkedIn profile, Haines "gained over 23 years of working experience in ever increasing positions of responsibility within private, NGO and military environments" which enabled him "to hone my skills in negotiation, programme management, operations and security to the highest standards".
This includes holding down "various" military positions between 1988 and 1999, before joining a business that helped rebuild the former Yugoslavia in the early '00s. He is currently a director of a catering supplies business but has also worked for aid organisations in war zones.
In 2012 he spent time working for the peacekeeping organisation Nonviolent Peaceforce in South Sudan as a "non-partisan unarmed civilian peace facilitator".
The organisation said in a statement that he was not working for it when he was abducted in Syria.
"We join with people around the world in pleas, thoughts and calls for his safe release," Nonviolent Peaceforce said. "NP in its pursuit of protection of civilians and violence reduction never makes political comment on nor demonises any individual or organisation."
His name initially went unreported at the request of his family.
Although journalists have known of Haines' imprisonment for some time, his name was kept out of the media at the request of his family. Following the release of the video, the Foreign Office continued to ask British outlets not to name Haines, but foreign outlets, including The New York Times, have done so.
According to The Times, the Foreign Office feared that naming him could increase his profile, and therefore, in the eyes of his captors, his potential ransom value. But the family concluded that since his name had already been published around the world, it would inevitably appear in the British media.