British police say their investigation into the London bombings is making rapid progress, and they have identified four suspects.They believe at least one of the men is likely to have killed himself in a suicide bombing. If confirmed, it would be western Europe’s first attack of this kind. The police were able to quickly gather enough evidence to lead to an arrest yesterday, and the seizure of explosives. Three of the four men came from West Yorkshire, and all are believed to have been British nationals.All of them travelled to London by train, meeting at King’s Cross station, about half an hour before the explosions happened. It is unclear if all died in suicide bombings, or if the person who made the bombs took part in the operation. If the suicide bombing thesis is confirmed, security experts agree that it is a new threat against which there is little defence. Police swoops in Luton and Leeds led to big security alerts; officers did not want a similar scenario to what happened in Madrid when police surrounded a seven-man terrorist cell. They blew themsleves up rather than be captured. In Leeds the homes of relatives of the suspects were raided, with one family member arrested and explosives seized; in Luton, explosives are believed to have been discovered in a parked car.