Londoners were preparing defiantly to get back to business Monday after the horror of last Thursday's quadruple bombings, amid palpable jitters of a repeat strike and a growing debate about how to keep terrorists at bay.

Officials believe the near-simultaneous rush-hour attacks on the Tube were not the result of suicide attacks but devices deliberately left in bags. A fourth attack on a bus may also have been planted. The implication is that the terrorists, thought to be a small cell of Islamist radicals, possibly Britons, are still at large.

Police and security services are mounting one of their biggest ever investigations, hoping that assiduous forensic work and hundreds of hours of closed-circuit television (CCTV) footage will pinpoint the perpetrators of what has been dubbed "7/7."

Experts note the similarities with the March 2004 Madrid train bombings. In that case, investigators found further explosives and signs of more terror plots, and cornered the gang in a violent confrontation in which the suspects blew themselves up.

"These people will be desperate to escape," says Shane Brighton, a security expert with the Royal United Services Institute, a think tank in London. "They are dangerous when being pursued."

But he added: "What seems to be the case with these Al Qaeda attacks is that when you have one of them, there doesn't seem to be another straight afterward."

Police are taking no chances on that front. Significantly, Spanish forensic teams are helping with the London inquiry. The wholesale evacuation of Birmingham city center on Saturday night, an alert at Heathrow Airport Sunday, and the heightening of the general terror alert to the highest notch indicated a society on edge.

"Our fear is that there will be more attacks until we succeed in tracking down the gang who committed the attacks," said Charles Clarke, the home secretary.

Intelligence officers within MI5 and MI6 are believed to be concentrating their focus on the electronic and airwave "chatter" that often precedes such events. The police effort, meanwhile, has concentrated on forensic investigations and CCTV footage. London is one of the most closely watched city on earth, with thousands of cameras above and below ground.

"There's no doubt that it can help," says Martin Gill, a professor in criminology at Leicester University. "The real advantage of CCTV is that is may be able to help track people throughout London and beyond to pick up evidence."

Two separate claims from groups saying they are affiliates of Al Qaeda have claimed responsibility for the attacks. A former police chief, Sir John Stevens, said the terrorists "will almost certainly be British born and bred," noting that it is believed that perhaps 3,000 Britons have passed through Al Qaeda camps.

Newspapers here are speculating of involvement by the North African cells thought to be behind the outrages of Madrid and Casablanca. And they're wondering if the attacks were timed to spoil the joy of the 2012 Olympics announcement, with a terror cell ready to hit Paris, if it, not London, had won the bid.

"The real debate would be about the type of network that is involved," says Prof. Paul Wilkinson, a terrorism expert at St. Andrews University in Scotland. "Is it homegrown or are there people from overseas who are helping to organize and coordinate it?"

The identity of the perpetrators and attack details are vital for a government desperate to make Britain a harder target for terrorists. Despite rigorous antiterrorist legislation, police had warned before Thursday that an attack was inevitable.

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