The Theories: All signs point to mass murder bid - but who did it and why did they fail?

CELL LINKED TO 7 JULY BOMBERS, TRYING TO COMMIT MASS MURDER

Given the apparent similarities with 7 July, police are working on the assumption that the bombers were trying a repeat operation. According to the Metropolitan Police Commissioner, Sir Ian Blair, it was an attempt to commit mass murder that went wrong.

There were no casualties and no obvious major event was upstaged (the previous attacks coincided with the G8 summit), but the timing was suspicious, exactly two weeks after 7 July. Both bombing waves were clearly co-ordinated, as the Tube explosions were almost simultaneous on both days. Yesterday, however, the attacks came at lunchtime, whereas on 7 July, it was at rush-hour.

Yesterday, three Tube stations and a bus in different parts of the capital were targeted again. On both days, the bus bomber was on the upper deck. The three Tube lines hit yesterday all pass through King's Cross, the station where the 7 July bombers had arrived from Luton. The bombers again used rucksacks. And in another striking similarity, the four attacks were loosely in the form of a cross on a map of London - just like 7 July.

Unlike two weeks ago, the bombers did not manage to bring the transport system to a halt for many hours. But they did sow panic among those going about their business in four districts of the capital.

The remnants of the rucksacks will provide forensic scientists with clues as to the type of explosives used. According to reports last night, the explosives were similar to those used by the suicide bombers. In a stroke of luck, some of the rucksack bombs may not have exploded, again providing clues.

While the 7 July bombings were admitted by a group calling itself the Secret Organisation of al-Qa'ida in Europe, no one admitted immediate responsibility yesterday. Documents found on the suicide bombers' bodies led police to Yorkshire, where they found a bath of volatile "Mother of Satan" explosives in Leeds. From there, the inquiry led to Pakistan, where some of the suicide bombers were thought to have spent time. It could be that the 7 July cell was part of the same network. Yesterday could signal the second phase of an orchestrated campaign.

On 7 July, the Government was swift to point out that the bombings bore the hallmarks of al-Qa'ida: no warning, well- co-ordinated, low-tech and intended to kill as many as possible. Yesterday, too, there was apparently no intelligence about the timing of the attacks and they also seem to have been well co-ordinated and low-tech. However, Sir Ian said that it was too early to point to a connection with al-Qa'ida.

CELL LINKED TO 7 JULY BOMBERS, AIMING TO SOW PANIC RATHER THAN KILL

Troubling questions remain for the police: if the attacks were designed to kill as many people as possible, why did the attackers use such small quantities of explosives? And given the professional way that the 7 July bombers went to their deaths, how is it possible that yesterday's bombers failed so miserably if they did intend to commit mass murder? Was the bomb-maker the same individual responsible for the devices on 7 July? If so, the chance of failure of all four are extremely low. Or was the bomb-maker for yesterday's attack inexperienced? Another theory would be that the detonators were faulty or there was some other mechanical malfunction in the devices.

If they wanted to sow panic, however, they succeeded, albeit at the risk of detection. For the first time in London, police wore chemical-weapons protection to investigate. The attacks caused Tubes, a bus and buildings in four London districts to be evacuated. Part of the Tube system was paralysed, adding to a network which is already heavily handicapped by the aftermath of the 7 July attacks.

Unliketwo weeks ago, the terrorists were not suicide bombers - although whether that was by accident or design is not certain. But they took a risk by leaving their rucksacks on the Tube and the No 26 bus, then running for their lives.

COPYCAT BOMBING BY ANOTHER ISLAMIST GROUP

There is no shortage of young, radicalised Muslims with grievances who may want to use the excuse of the 7 July attacks to launch a copycat bombing. Chatham House, the London think-tank, warned this week that recruitment and funding for al-Qa'ida had been boosted by the Government's support for the war in Iraq.

Some radical Muslim preachers have also failed to condemn the suicide bombings. Police have suggested that up to 200 potential bombers may be in Britain. But it is uncertain whether they would have access to the organisational and technical resources necessary to carry out an attack within two weeks of the 7 July bombings - that, of course, may explain why the bombs never exploded.

COPYCAT BOMBING BY NON-ISLAMIST GROUP

Again there is no shortage of groups with a history of carrying out explosive attacks who may have been tempted to pile in. But such attacks are usually claimed: animal-rights extremists claimed responsibility for an arson attack this week on an Oxford University boat-house that caused an estimated £500,000 of damage.

Some security specialists are already expressing fears that "every half-baked terrorist in the country" would be considering launching an attack. However it is very unlikely the attack was the work of anyone other than Muslim extremists.