British Home Secretary Charles Clarke warned on Friday the bombers who attacked London's transportation network could strike again.
Speaking to British Broadcasting Corp radio, Clarke said "We have to have ... maximum consideration of the risk of another attack, and that's why our total effort today is focused on identifying the perpetrators and bringing them to justice."
"That is of course the No. 1 preoccupation that the police and security services have at this moment," he said.
Clarke said police have not ruled out the possibility that one or more of the attackers who struck three subway trains and a double-decker bus on Thursday could have been suicide bombers.
"No particular hypothesis has been ruled out or confirmed," he said.
Four apparently coordinated bomb explosions hit central London Thursday morning, killing at least 37 people and injuring some 700 others.
Also on Friday, the home secretary rejected any idea of an intelligence failure on the attacks, saying the bomb attacks came "out of the blue."
Clarke said "as far as the general threat assessment was concerned, we didn't have prior knowledge of this attack."
"We obviously are looking very carefully at all our intelligence to see if anything was missed, but in fact we don't believe anything was missed, It just came out of the blue," he said.
British newspapers had earlier questioned the alertness of Britain's security services.