The copycat attacks on London suggest that the terrorists behind the July 7 blasts are still at large and intent on causing havoc and bloodshed, according to terrorism analysts.
Robert Ayers (search), a security analyst at Chatham House in London, said that he believed that the same group was behind both attacks.
"All along I've been saying that you had four guys that died [in the July 7 bombings], but the infrastructure that trained them, equipped them, funded them, pointed them at the right target — the infrastructure's still in place, still here," he told the Reuters news agency.
But if the same group was involved, the obvious question was why the first wave of attacks was so professional and deadly, and the second was apparently so amateur, continued Mr. Ayers, a former U.S. intelligence official.
He pointed out that police had recovered unused explosives from various sites, including a rental car abandoned by bombers at Luton (search). Police carried out 10 controlled explosions on the rental car in the Luton station parking lot before they placed it on a flatbed truck and took it away.
"One speculation I've had all along is that they left those explosives in the car for another group to pick up and carry out a second attack, but when they got there the car had already been taken over by the police, so they have had to cobble something together fairly quickly," he said.
"From what I've been able to gather, either the bombs themselves are very, very small compared to two weeks ago, or they've got a manufacturing problem and only the detonators are going off, and not the primary charge. They're certainly using explosives that aren't nearly as powerful."
Experts agreed that there were two explanations for today's attack. The first, more benign, is that the attacks were carried out by "imitative amateurs" inspired by the July 7 blasts. The second, more worrying, was that the same group behind the suspected Al Qaeda-linked attackers had struck again.
That would show that, far from exhausting its strike potential, the group was capable of causing fresh havoc despite heightened security precautions and a high state of alert among both the police and public. It also would show that the group could readily mobilize fresh operatives — perhaps even would-be suicide bombers — to follow the example of the four bombers who blew themselves up.
Michael Clarke, a security expert at King's College London (search), told Reuters: "The more we know about the bomb attack two weeks ago, the more skillful it looks, well planned — the people behind it know what they're doing."
Prof. Paul Rogers, of Bradford University (search), agreed that the second wave of attacks was an "ominous" development. He said: "It implies there might be another cell primed and ready to attack. The one ominous thing is that this appears to be a group of a similar nature to the previous July 7 bombers."
Prof. Rogers said, however, that the apparent failure of the devices to detonate on the Underground lines would provide investigating teams with crucial evidence for the earlier attacks.
He said: "The level of forensic evidence will be extremely high, much higher than last time. They will have the devices and much can be done to them in terms of fingerprinting, DNA, the origin of the detonators and where the bags were bought. If this was a series of dummies deliberately timed to cause mass panic, then it puts the people responsible at considerable risk of being found."
Professor David Capitanchik, a terrorism expert based at Robert Gordon University in Aberdeen, Scotland, warned that today's explosions may have been "amateurish" devices deliberately aimed at emergency services who arrive at the scene. He said eyewitness reports of small explosions in a backpack could indicate that they were not intended to explode properly until they had been recovered.
"It appears as if the detonators have gone off, but reports indicate a much more amateurish-made device than the bombs two weeks ago," he said.
"In other parts of the world which have experienced incidents like July 7 in London, smaller bombs have later been put in places with the hope that they will go off when the police and emergency services examine them. This indicates that in today's situation the police are going to take a great deal of time and exercise great care, as there is a possibility that these bombs are intended to entrap police and emergency workers."
All were agreed, however, that this Thursday's attacks would add to the fear, which had been beginning to subside. Mr Ayers said that although casualties appear to have been avoided, the long-term damage to the city's psyche may take even longer to heal.
Mr. Clarke said: "It is entirely plausible that they will have planned a campaign, not just one bomb. It's part of terrorist psychology that one bomb is never enough.
"You gain the effect that you want by creating a sense that there are lots of bombs and the public are going to have to live with this for a long time, unless they do something, unless the government changes. The second event is a prerequisite to the psychology of a campaign ... This important because it's momentum for terrorists."
Former government intelligence officer Crispin Black agreed that those who planned the attacks were trying for the maximum psychological effect.
Speaking outside the police cordon surrounding Warren Street tube station, he said: "It could be that this is a nasty sort of copycat attack mimicking what happened two weeks earlier but not using quite the power of explosives, but still getting the chaos and fear effect as you can see around us.
"In this stage of a counter-terrorism campaign you're bound to get the feeling that rings are being run around us."
Mr. Black said this will be the case until intelligence on all the attacks improves. But he said incidents like today should help police catch the perpetrators.
"In this kind of attack it looks as thought the terrorists have put their heads above the parapets, and that falls into Scotland Yard's hands," Mr Black said.
"It then becomes much easier for police to start tracking them down."
He added: "It's too early to tell who these people are, but even if they're just copycat attackers, that's a pretty depressing thing to think about, that there are young people out there who are so radicalized they are prepared to bring London to a kind of taunting halt. And that's the best scenario."