NEWPORT, Wales (Reuters) - British police arrested three men in south Wales over last week s bombing of a busy commuter train in west London, meaning five people are now being questioned by detectives over the attack which injured 30 people.  A 25-year-old man was arrested on Tuesday evening in Newport, while two others, aged 48 and 30, were detained at another address in the Welsh town in the early hours of Wednesday, London police said.  This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday,  said Commander Dean Haydon, head of London police s Counter Terrorism Command.  We now have five men in custody and searches are continuing at four addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack.  The home-made bomb went off on Friday during the morning rush hour on a packed train at Parsons Green underground station, engulfing the carriage in flames, although it appeared that the device did not fully explode.     In the aftermath, the authorities raised Britain s threat level to its highest rank of critical, meaning an attack was considered imminent, but lowered this after arresting two men on Saturday.   An 18-year-old was arrested in the departure lounge at the port of Dover on Saturday and another suspect, 21, was detained hours later in the west London suburb of Hounslow. Both were said by local media to have links to a property in Sunbury-on-Thames, a town just outside London, where police were carrying out a major search.  The house belongs to a couple who have fostered hundreds of children, including refugees. The leader of the local authority was quoted as saying the 18-year-old was an Iraqi who had come to Britain as an orphan.  Islamic State militants said they were responsible for the attack although both British and U.S. officials have cast doubt on the claim, saying there was no evidence any recognized militant group had ordered or organized the bombing. Friday s bombing was the fifth major attack regarded by authorities as a terrorist incident in Britain this year which have claimed the lives of 36 people. 