Initial tests have revealed marked similarities between the devices used on both occasions and also evidence that it was only faulty detonators that prevented carnage yesterday. At least three of the men responsible for yesterday's attempted bombings were on the loose last night after fleeing the scenes.
Questions were raised about what appeared to be another failure of intelligence. The security agencies had been caught out by the 7 July bombings which, Home Secretary Charles Clarke admitted, "came out of the blue". Yet yesterday bombers with rucksacks, just like last time, managed to get to their targets undetected.
And yesterday was the day that the Prime Minister met senior officials from the police, MI5, MI6 and GCHQ (the Government's listening-post) in what was in effect an inquest into what went wrong two weeks ago. But police and the security agencies stress that this time they have crucial advantages lacking in the previous bombings. Unexploded devices will yield vital forensic clues. The Metropolitan Police Commissioner Sir Ian Blair said: "We believe this may represent a significant breakthrough."
Sir Ian added that the attempted blasts on three Tube trains and a bus, a copy of the 7 July attack, "must have been to kill". He said he was not in a position to state whether the two attacks were connected, but he added: "There were four attacks and there were four attacks before. We can all see that the attacks have a similar pattern."
Detectives are waiting to question a man believed to have been injured in the attempted attack at Warren Street. Witnesses said they saw a man running into the nearby University College Hospital. Armed officers entered the hospital and after 4.20pm a police convoy left at high speed with motorcycle outriders.
Two men were arrested in the afternoon, including one at Whitehall, who was ordered to the ground by armed police. But police said that they were not linked to the attacks.
Passengers who had streamed out of the trains and the No 26 bus at Hackney Road, in Shore ditch, east London, were corralled by police into makeshift interview areas for statements and to check whether they had taken any images with their mobile telephones. Officers also began to study CCTV footage.
Unlike the 7 July bombings, by suicide bombers prepared to sacrifice their lives, the terrorists yesterday abandoned their bombs and fled, pursued at least on one occasion by passers-by.
The detonators on at least two of the devices are believed to have initiated but did not ignite the explosive in the rucksacks. One did not detonate at all, and another only partially. In all four, the main charges were not detonated. Sir Ian refused to confirm or deny reports that the devices were packed with nails and shards of metal.
It is still unclear what type of explosive was used in the bombs. One of the key questions is whether the material used in the explosive is the same homemade material used in the bombs of 7 July. Police are still struggling to discover the make-up of the explosive used two weeks ago, but it is thought to be of a type similar to material known as the "Mother of Satan", triacetone triperoxide or TATP. The base ingredients - drain cleaner, bleach and acetone - can be bought easily without attracting suspicion; its chemical composition is simple, and in its finished form it is almost undetectable by sniffer dogs or conventional bomb-detection systems.
The detonators used in yesterday's attack appear to be the weak point. Andy Oppenheimer, an explosives expert with Jane's Information Group, said the "popping" sound heard by witnesses could indicate the explosion of a detonator.
"That would be a typical sound that would be heard, a dull thud. This type of malfunction used to happen to the IRA, who had expert bombmakers. It could be bad workmanship or it could be a deliberate attempt to disrupt the city two weeks later."
There had been reports that the staff at University College Hospital had been told to look out for a man of black or Asian appearance wearing a blue top who had run into the hospital. But this was later denied by hospital authorities.
However, three rooms in an unoccupied part of the hospital remained cordoned off last night. In a statement, the hospital said: "Police have confirmed that there is no danger to patients or staff. Armed police conducted a search of the hospital this afternoon and this has been completed. Three small rooms in an unoccupied part of the hospital have been cordoned off and police inquiries are still continuing.
"We are cancelling routine appointments and surgery tomorrow for patients not already in the hospital.''
Robert Naylor, the hospital trust's chief executive, said: "Our first priority has been the safety of patients and staff. Throughout this afternoon we have worked closely with the police in their investigations while maintaining emergency services."
* The hunt for members of the terrorist cell behind the 7 July bombings was mired in confusion last night after it emerged that no one had been arrested in Pakistan in connection with the London attacks. The British high commissioner in Islamabad rejected claims that the "al-Qa'ida mastermind" of the attack had been captured.