LONDON (Reuters) - British police said a bomb was used during an explosion at a London metro station which injured 18 people in what officers described as a  terrorist incident.   We now assess that this was a detonation of an improvised explosive device,  Britain s top counter-terrorism officer Mark Rowley said on Friday.  London s police is being supported by Britain s MI5 intelligence service, he said. 
London metro station incident caused by bomb, top UK police officer says