So far there is little direct evidence to link the plots and police are treating them as separate inquiries.
Although there are reports of links via the Finsbury Park Mosque or white-water rafting trips in Wales, police have found nothing substantial to go on. Reports from the interrogation of Osman Hussain in Rome say he has denied a link and the 21 July attempted attacks were, in essence, a "copycat" exercise.
Although both groups used similar variants of a home-made explosive, acetone peroxide, it is a widely known method, used by other terror groups, easily available on the internet.
Is there a mastermind behind both plots?
It remains likely that others helped guide and prepare both cells and possibly other cells who could prepare more attacks. Whether it is one or more persons and where and how they might be linked is unknown; it is possible that any guidance came via the internet from another country.
Some have pointed the finger at Haroon Rashid Aswat, 30, a Muslim from the same part of Yorkshire as the 7 July bombers. Mr Aswat was arrested in Zambia last week as a result of a request by the US authorities who believe he was involved with an abandoned plan to establish a terrorist training camp in Oregon. Scotland Yard has downplayed suggestions he is a key figure.
Are there more cells?
Yes, according to Peter Clarke, the head of the anti-terrorist branch, who said that the threat remained "very real". Scotland Yard has not commented on reports that last Thursday's presence of armed officers on the streets of London was in response to a specific threat and not just public reassurance.
Police are hoping the large amounts of evidence they have accumulated may lead them to other cells. But they also know that, if there is a bigger plan being manipulated by a mastermind, then they must also look for a cell from another ethnic Islamic group, anywhere in the country, who are planning an attack by entirely different methods. And one that could come at any time.
Where does al-Qa'ida come into this?
It is unlikely Osama bin Laden or his associates ordered the London attacks, but it cannot be unequivocally ruled out, so little is known about the organisation. What is more likely is that all those involved, the bombers and those helping them, broadly sympathise with the aims and objectives of al-Qa'ida while not necessarily belonging to any organisation or having any direct links.
How was one man allowed to flee the country?
Questions will certainly be asked as to how Osman Hussain whose picture was broadcast worldwide on 22 July, could have slipped out of the country on a Eurostar five days later, en route for Rome where he was planning to hide with his brother. His photograph was the most blurred of the four released but was posted up at Waterloo, where his British passport was examined by French officials. Although regular checks by British immigration officials were stopped last year, spot checks were being conducted because of the alert and Special Branch officers were at the station. There also remains the possibility that police were following him, hoping he might lead them to others.
What about all the others who have been arrested?
As well as the three suspects for the 21 July bombings, Scotland Yard is holding the brother of one of the men. In Rome, the fourth suspect is facing extradition and two other men, reportedly his brothers, are also being held.
Additionally, more than a dozen other people arrested at various addresses in south London, Birmingham and in Sussex, are also still being questioned, but some are believed to be of less interest to the investigators. Two men arrested on a train at Grantham in Lincolnshire and two women detained at Liverpool Street Station have been released.
What do we know about the explosives? And was there a bombmaker?
Very little has been learnt by the police and even less officially released. That is despite the fact police have made several finds: a large quantity of home-made explosive in a bath at the house in Leeds used by the 7 July bombers, a cache of 16 components, including packed explosives and one ready-made nail bomb in a car in Luton, the five partially exploded bombs - including another nail bomb - left behind after the failed 21 July attacks and a quantity of materials discovered at a flat used by two suspects at New Southgate.
Although the chemicals and methods for making such explosives are readily available in the high street, the use of detonators and the introduction of other chemicals to stabilise the explosive points to more experienced hands.
It is known the bombs were variants on a mixture of acetone peroxide, also known as TATP or "Mother of Satan", but police have not yet, it is believed, been able to establish exactly what mix of chemicals was in the volatile material - it took six days just to stabilise the explosives found in Leeds. But the fact both cells used the same basic formula and both had nail bombs is not evidence of a link.
One fact that does point to all the explosive being made together just before 7 July is that the 21 July bombs failed to detonate because it had degraded - TATP goes "off" after two or three weeks.
Magdi Mahmoud el-Nashar, 33, an Egyptian-born chemist who completed a chemistry doctorate at Leeds University in May was arrested and questioned in Cairo because of alleged links with the Leeds bombers. But he denies being involved and the Egyptian government has said there is no link between him and the bombings.
Is there any doubt the 7 July bombers intended to kill themselves?
Not really, say police. But some intriguing questions remain. The men left behind a car at Luton station containing explosives, bomb-making equipment and one nail bomb - did they intend to return or were these left for others? Two had wives, children and stable jobs. All four could have believed they were simply going to set off timers and leave the trains at the next stop.
There are also no reports that they did any of the other traditional acts that such bombers do before entering paradise - such as shave their body hair or cry "God is Great" at the point of detonation. And why did they not leave behind some kind of message or video?
Is there a shoot-to-kill policy in London?
Police have confirmed the Operation Kratos guidelines introduced after 11 September 2001 and based on Israeli experience. It allows armed officers the right to shoot suspects in the head - rather than the body - to instantly disable and prevent a device being detonated by their weapons or a suspect's reflex.
The guidelines, it has been reported, also allow officers to shoot without identifying themselves verbally or otherwise or challenging a suspect to stop - absolute prerequisites in armed operations until now. Police say the policy is, essentially, "shoot-to-kill-if-needed" and say it is the only effective means of dealing with the threat.
How extensive is the police investigation?
It is, said Sir Ian Blair, the Metropolitan Police Commissioner, the greatest challenge and the biggest inquiry faced by the force in modern times. About 1,000 officers are working on the detailed hunt, supplemented as needed by armed squads. Dozens are believed to be working undercover - both on the transport systems and other key locations. SAS officers are also on hand in London.
Additionally, substantial resources are being devoted to the inquiry by MI5 and MI6, while the electronic surveillance centre at GCHQ is working in tandem with America's National Security Agency.
Jean Charles de Menezes
A 27-year-old electrician from Brazil, he had been living in this country for about three years. He was shot dead by police at Stockwell station on 22 July. According to police, surveillance officers had followed him from a flat in Tulse Hill and called in armed support as he approached the station. When they challenged him outside the station, he fled. He was pursued onto a Tube train and shot eight times. The police admitted he had nothing to do with the bombings. The killing is being investigated by the Independent Police Complaints Commission, which will have to resolve several key issues:
Why was he followed at all?
He was living in a block of flats with a communal entrance. Police were reportedly watching a different flat from the one he lived in. The Commission will want to know what information the officers had and why they followed him. They will also need to clarify why, if they believed him to be a bomber, he wasn't stopped when he boarded a bus. Police and the MoD say the SAS were not involved, but there have been reports the SAS provided technical help with the surveillance.
What was he wearing?
Although police initially said Mr de Menezes was wearing a "bulky coat" in warm weather - an indicator of a potential suicide bomber in recently issued guidance given to officers under the nickname Operation Kratos - his family now say they have been told he was wearing a lightweight denim jacket.
Was he challenged?
Crucially, the Kratos guidelines allow officers not to identify themselves and not to "approach, challenge and/or negotiate" with a potential bomber, in case he or she sets off a bomb. Although police have said Mr de Menezes was "challenged", that appears to have contravened their own guidelines and no one has reported that they heard a warning.
Did he run away?
Witness accounts initially seemed to support the police version that Mr de Menezes ran away when challenged. But one man who said he saw someone leaping the ticket barriers has told the Daily Mail he now believes that to have been a police officer. Also, according to his family, police have now told them their son did not jump the barrier. The IPCC may find some of the answers in close-circuit television footage.
Why might he have run?
Mr de Menezes was, according to witnesses, running by the time he reached the Tube train. There are several overlapping possible explanations for this. If he did believe he was being pursued by police, he might have been fleeing because, according to the Home Office, he was living in Britain on an expired student visa, with a forged stamp on his passport. In Brazil, police regularly shoot suspects, so he might have feared a similar fate. But his friends have also suggested a simpler explanation: that he did not hear a challenge - or one was not delivered.
Mr de Menezes was buried in his home town of Gonzaga last Friday afternoon. About 8,000 people lined the streets.
* Hasib Hussain, 18, killed 12 people in a bus explosion in Tavistock Square. He was a youthful tearaway said to have been radicalised on a trip to Pakistan in 2003. Arrested for shoplifting in 2004. Where was he during the "missing hour" between the Tube explosions and his bomb being detonated? Did he really intend to detonate the bomb?
* Mohammad Sidique Khan, 30, killed six at Edgware Road. Oldest of group; married with one daughter. He knew Hussain and Tanweer through the Muslim communities of West Yorkshire. Travelled to Karachi with Tanweer last year. Unconfirmed reports linking him to Finsbury Park mosque. Was he the ringleader? What were his connections to radical clerics?
* Shahzad Tanweer, 22, killed seven people on Circle line train at Aldgate. A sports science student from Leeds Metropolitan University, his family run a fish and chip shop in the Beeston area and have said they were "shattered" by his involvement. He was arrested for disorderly conduct last year, and later travelled to Pakistan with Khan.
* Germaine Lindsay, 19, killed 25 with bomb on Piccadilly line near King's Cross. A carpet fitter from Aylesbury. From a Jamaican background, he converted to Islam and married a British woman, also a convert; she is pregnant with their second child. Met up with other bombers at Luton, but how he first became involved with them remains unknown.
* Muktar Said Ibrahim, 27, suspected of attempting to detonate bomb on bus in Hackney Road, Shoreditch. Eritrean-born, he joined refugee family in north London in 1992. Convicted in 1996 of violent mugging, he may have been radicalised in young offender institutions. Given British citizenship last November. Arrested in Dalgarno Gardens, North Kensington, west London, last Friday after brief siege.
* Yasin Hassan Omar, 24, accused of trying to bomb a Tube train at Warren Street. A Somalian refugee who came to Britain aged 11, fostered, then given a flat at Curtis House, New Southgate in north London, which he shared with Muktar Said Ibrahim; rent paid by council. Both said to have attended Finsbury Park Mosque. Arrested in Birmingham last Wednesday after being stunned with a Taser.
* Ramzi Mohammed, age unknown, accused of trying to detonate bomb on Underground between Oval and Stockwell stations. Other travellers tried to detain him when smoke came from his rucksack, but he escaped. Like other 21 July suspects, believed to be from east Africa. Reportedly a bus driver on the route which runs from East Acton to Russell Square. Arrested with Muktar Said Ibrahim in Dalgarno Gardens.
* Osman Hussain, 27, is accused of attempting to set off a bomb at Shepherd's Bush Tube station. Caught in white vest on camera on bus, having discarded his shirt. Lived in Stockwell, south London, with wife and three children. Escaped before police raided home last Wednesday, but traced via mobile phone to brother's home in Rome. Arrested by police on Friday; extradition may take weeks or months.
* Wahbi Mohammed, 23, brother of Ramzi Mohammed, will be questioned about whether he is the so-called "fifth bomber" who dumped a partially detonated nail bomb in a rucksack discovered on 23 July at Little Wormwood Scrubs, a short distance from Dalgarno Gardens. He was arrested after a raid on another property less than a mile away in Tavistock Crescent, Notting Hill, on Friday morning.