london british detectives arrest 17-year-old youth connection bomb attack underground train london last week injure 30 people bring total number arrest six police say thursday young man arrest early hours thursday thornton heath south london arrest take place dover south coast england hounslow west london newport wales home-made bomb go sept. 15 morning rush hour pack underground tube train parsons green station send flame carriage although appear device fully explode fifth major terrorism incident britain year continue fast-moving investigation significant amount activity take place since attack friday say commander dean haydon head metropolitan police counter terrorism command six males custody search continue five address detectives carry extensive inquiries determine full facts behind attack say men custody south london police station 18-year-old 21-year-old 25-year-old 30-year-old 48-year-old british police make sixth arrest tube bomb investigation