The suicide cell that killed 52 people on 7 July is not linked to those alleged to be behind the second London attacks on 21 July, according to the initial findings of the biggest anti-terrorist investigation held in Britain.
An investigation into the four suicide bombers from the first attacks and the people alleged to be behind the July 21 plot has found no evidence of any al-Qa'ida "mastermind" or senior organiser. The inquiry involved MI5, MI6, the listening centre at GCHQ, and the police.
The disclosure that the July 7 team were working in isolation - and were radicalised by Mohammad Sidique Khan, the oldest man - has caused concern among anti-terrorist officers.
Police and MI5 fear it increases the chance that more "self-sufficient" units similar to the July 7 suicide cell are hiding in Britain. Anti-terrorist officers are worried by the evidence that previously unknown "clean skin" terror cells are forming in Britain with little or no help from abroad.
The alleged plotters behind the July 21 bomb incidents in London are thought to have been "copycats", targeting Tube trains and a bus.
The intelligence assessment was made in the past few days. "The key point is that the events are not connected," said one counter-terrorist source. "It appears they were self-contained, rather than being organised by some kind of mastermind.
"It is concerning that none were on the intelligence radar. There are quite probably others we do not know about out there. Over the past 10 years, we have been successfully disrupting a number of groups of people who could have carried out bombing attacks similar to those we have seen in the past few weeks."
"We can't disrupt them all. They only have to be lucky once - and they have been. At some point there will be another suicide or bombing group."
The intelligence agencies and the police have been trawling through telephone and computer records, e-mails, forensic evidence, and investigating friends and associates to build up a picture of the suicide bombers.
They have found that the July 7 cell, three of whom were of Pakistani background and came from the Leeds area, while the fourth was living in Buckinghamshire, did not conform to previous al-Qa'ida units.
A police source said: "All the talk about 'Mr Bigs' and al-Qa'ida masterminds looks like something from a film script at the moment. Of course, things could change if new intelligence comes through, but it looks increasingly as if these people were largely working on their own. It is not something we expected."
Meanwhile, an Egyptian chemist from Leeds who admits knowing two of the four-man suicide team - and left Britain a week before the attack - is still being investigated as a possible bomb-maker. The police are waiting for the results of forensic tests to discover whether his fingerprints or DNA was among the explosives and equipment found in a Leeds bomb-making factory and in a hire car used by one of the terrorists.
Magdi Mahmoud el-Nashar, the chemist, was released from custody in Egypt earlier this month, after three weeks of questioning by the police. Egyptian authorities said they found no evidence to link the former Leeds University student to the attack.
Intelligence officers now believe the four British-born suicide bombers - Shahzad Tanweer, 22, Hasib Hussain, 18. Mohammad Sidique Khan, 30, and Germaine Lindsay, 19 - were probably organised and radicalised by the eldest bomber, Khan. The 30-year-old teaching assistant who, like Tanweer, spent three months in Pakistan before returning to Britain in February this year, may have been instructed in bomb-making techniques at a foreign camp. Alternatively it is thought that he could have been assisted in Britain or obtained information from the internet.
Senior police sources in West Yorkshire suggest that gyms and boxing clubs in Leeds - rather than mosques - were the key to the development of the young men into bombers.
It was at a gymnasium in Lodge Lane, Beeston, that Khan is thought to have begun radicalising the two younger Leeds-based men - Hussain and Tanweer.
Already an accomplished youth worker of 10 years' experience, he appears to have brought both Hussain and Tanweer to a gym established in the basement of the Hardy Street mosque, also in Beeston.
Khan was eventually forced to leave the gym at the Hardy Street and he set up another gym at the former Hamara youth centre in Lodge Lane, where he was noted for not allowing adults in while the boys were training. One of the remaining mysteries of the July 7 bombings is the link between Lindsay and the other three attackers. Lindsay hailed from Huddersfield, 20 miles away and, unlike Tanweer and Khan, he was not known to Hussain's family. Yet Lindsay's telephone number was stored in Hussain's mobile. Evidence of a recent link between Lindsay and Hussain is provided by Mr Nashar, who described how he and Lindsay met last October at the Leeds Grand Mosque, five miles from Beeston, where Lindsay had asked him to find him somewhere to live. He says he introduced Lindsay to the flat that eventually became the attackers' bomb factory.
* A memorial service for the victims of the July 7 bombings will be held at St Paul's Cathedral in London on 1 November.