British police are claiming “significant” progress in their hunt for the July 21 London bombers, with the investigation centred on a flat in the north of the capital and its surroundings, where at least two of the quartet spent some time.A garage near the block of flats is being studied and chemicals and bomb-making equipment found there is being analysed. More details have also been released of the two named attackers, one of whom was identified by his parents. Muktar Said Ibrahim, born in Eritrea, was known to police, having been jailed as a 17-year-old for five years for violent assaults on commuters. On release, he is alleged to have met “shoe-bomber” Richard Reid, who is now serving a sentence of life imprisonment in the United States. Ibrahim is a British passport holder and a naturalised citizen. Yasin Omar, a Somali national, is in Britain on an unlimited residence permit. A car parked nearby is also being examined after traces of explosives were discovered. Residents say the two men returned to the flat after their failed attack and, it is feared, they removed explosives before disappearing. Police also say they are now convinced a fifth bomber is on the loose and passport checks on everyone leaving the country have been reimposed.