Britain's top law enforcement official has said the bombers who hit London's transportation network could strike again.
"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," Home Secretary Charles Clarke told British Broadcasting Corporation radio on Friday.
"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 Underground trains and a double-decker bus on Thursday could have been suicide bombers.
"No particular hypothesis has been ruled out or confirmed," he said.
Claim for attacks
Clarke said investigators were examining a website claim in which an organisation calling itself the "Secret Group of al-Qaida's Jihad in Europe" said it was behind the attacks. That claim could not be immediately verified.
"Their claim is something we certainly take seriously," he said.
Home Secretary Charles Clarke
spoke to BBC radio on Friday
The group called the bombings punishment for Britain's involvement in the war in Iraq and invasion of Afghanistan.
It said Italy and Denmark were being warned for their support of the US-led coalitions in both countries.
Clarke said it wasn't known whether the bombings were revenge for Britain's involvement in Iraq.
"There's no evidence as of yet that it had anything to do with the Iraq war as such," he said. "Of course, it may have. We'll have to see."
He said he didn't know whether the attackers were Muslim terrorists.
"It's very possible that that was the case, but we haven't ruled out other forms of terrorist attacks," he said.