An image grab taken from a video released by the Islamic State and identified by private terrorism monitor SITE Intelligence Group on September 13, 2014 shows a masked militant threatening to execute British hostage Alan Henning as his next victim just after beheading 44-year-old British aid worker David Haines.br br The Islamic State jihadist group had on Saturday released a video 2minute and 27 second showing the execution of British aid worker David Haines.