**EDITORS PLEASE NOTE: THIS EDIT CONTAINS GRAPHIC MATERIAL -- DOES NOT SHOW BEHEADING--<br /> <br />A friend and former colleague of gruesomely executed David Haines tears up as she searches for words to describe the 44-year-old father of two.<br /> <br />Islamic State militants released video Saturday that purports to show the beheading of Haines.<br /> <br />Dubravka Kukic says goodbye to the man she once called a friend.<br /> <br />(SOUNDBITE) (Croatian/English) DAVID HAINES' FORMER COLLEAGUE, DUBRAVKA KUKIC, SAYING:<br /> <br />"And it goes like this: 'It is amazing how can you speak right to my heart, without saying a word you can light up the dark', I salute you, my friend, I salute your bravery and kindness, I will miss you, rest in peace."<br /> <br />A second British hostage named Alan Henning is now under threat. The militant group says he will be killed if the United Kingdom continues its fight against Islamic State.<br /> <br />British Prime Minister David Cameron says the UK "Will hunt down those responsible and
