I don't know where to begin. How could a key "person of interest" (if not also a witness or suspect) be allowed to leave the country? What kind of defence is it where the answers he gave might incriminate the person giving them and so he is exempted? The eldest brother was involved to the extent that a) his driving licence was used because his brother was too young to hold one), b) certain IS propaganda material was found on his phone, c) he was photographed holding a heavy weapon with an IS logo in the image.
(Incidentally, both the father and mother of the suicide bomber have already returned to Libya.)
I smell a great big fat stinking MI6 rat. Any comments?