Britain's minister for law-and-order says the country should be prepared for more attacks like the apparent suicide bombings of London's transport system last week.
Home Secretary Charles Clarke has told British radio that the public must assume more potential terrorist bombers are living in their midst.
"I certainly think we have to organize ourselves on the basis that there are other people prepared to act in this way and we have to protect ourselves in the best way that we can against that," he said. "And we have to assume there are others who are ready to do the kind of things these people did last Thursday."
He spoke as police search for the masterminds behind the bombings that killed more than 50 people and wounded 700 others on three subway trains and a bus in central London.
Authorities say they have identified three of the four bombers. London media say they are British-born men of Pakistani decent, ranging in age from 19 to 30. Police believe a fourth bomber also was killed.
The focus of the police investigation has shifted toward finding out who may have recruited, financed and organized the bombers, and who provide them with the military-style explosives they used.
A relative of one of the suspects is in police custody after being arrested during a series of raids in and around the northern city of Leeds, where three of the suspects lived.
Home Secretary Clarke says he was surprised and shocked to learn that British-born Muslims were involved in the attacks, and he is appealing for Muslim leaders to display courage and leadership to root out fundamentalists in their community.
"It's a question of saying, 'We have to defend the values of society against those who would seek to destroy it,' " he said. "And that means standing out against, in a very strong way, anybody who preaches the kind of fundamentalism which can lead four young men to blow themselves and others up on the Tube on a Thursday morning."
Mr. Clarke says he will be considering extra measures he can take, such as deporting foreigners who advocate violence.