Were the four bombers being controlled or acting alone?
Why did they buy return tickets if they were intending to die?
Could MI5 have found out what was happening and stopped it?
Was an al-Qa'ida suspect allowed to travel to the UK unwatched?
The unanswered questions of the London attacks.
Suicide bombers do not buy return tickets. Theirs is a one-way trip. When four young men met at Luton railway station a week ago last Thursday, however, they gave every impression of going to London and coming back. They paid and displayed, leaving valid tickets on the windows of a Nissan Micra and a Fiat Bravo in the station car park. They boarded the 7.48am to London carrying return tickets.
Why would they do that, if they knew they would be dying very soon? The car park can be explained: perhaps they did not want to attract attention or get stopped. But the question of the train tickets has no obvious answer, unless the bombers were not aware that they would be among the casualties at Aldgate, Edgware Road, King's Cross and on the No 30 bus. They may have thought that they could leave their deadly bags on the train or the bus and walk away, merging safely into the crowd by the time a detonator set off the plastic explosive they called Mother of Satan to kill and maim in those enclosed spaces. Or were they told the bombs would go off later than they did?
The police think not, and reject suggestions that the men were duped, but such questions remain as they and the security services try to piece together what happened on 7/7 and - crucially - what it means for the future. Will it happen again, soon? Have other terrorist cells been activated now? Were the men really working alone?
Three of the men who stepped off the train at King's Cross Thameslink station at 8.20 in the morning of 7 July were from Beeston in Yorkshire, where tidy terraces are punctuated by boarded-up homes and money is scarce. They had driven down to Luton in the very early hours in a hire car.
The youngest was Hasib Mir Hussain, a tall, bulky young man of 18. He had got into fights with local boys in the past, and been cautioned for shoplifting, but otherwise was known for his love of sport and an evolving passion for his faith. Four years ago, after the attacks on the World Trade Center in New York, he had started to change. Some former fellow pupils at the then Matthew Murray Comprehensive claim he handed out leaflets justifying 9/11, but family friends say no, the changes in him were more subtle than that. "His family had an idea he was involved in a different kind of Islam from theirs, but I don't think they realised the depth of his conviction," said one. Lately, though, Hussain had become isolated from his family and friends, not least because he was avoiding the local mosques and travelling to a Bangladeshi one in Dewsbury instead. That was a big deal in a community so self-contained.
Dewsbury was where Mohammed Sidique Khan lived. He was the oldest of the four men who met that morning. The 30-year-old had been brought up in Beeston, where he was known as Sid, but he moved south after meeting Hasina Patel at Leeds Metropolitan University and marrying her. He still kept up links with Beeston, working as a classroom assistant and mentor to immigrant children in a local school, Hillside. He was patient, said the staff, and great with the kids. But there was trouble at home. As Khan became increasingly serious about his religion he clashed with his wife, who had far more liberal views about family life. The marriage, which had not been arranged by the families, was not approved of by his mother-in-law.
The couple split up a year ago, just before Khan went on sick leave in September 2004, said to be suffering from depression. He finally resigned from the school in December. Visits to his wife and 14-month old daughter Maryam became less frequent, although as he made his journey to London 10 days ago his wife was heavily pregnant.
Beside him was Shahzad Tanweer, 23, the funny, sport-loving son of a man from Faisalabad in Pakistan. His father, Mumtaz, was a former special constable and now a respected member of the Hardy Street mosque, who ran a fish and chip shop called South Leeds Fisheries. After studying sports science at Leeds Metropolitan University, like the others Tanweer had visited Pakistan to see relatives and study the Koran. He attended Markaz-e-Dawa, a madrassa or religious school in north Pakistan for three months, ending in February, but came back early saying he didn't like the people. His nickname in the family was Kaka, or baby.
Filmed by the closed-circuit cameras at the station, the four men looked, the police would say, like "happy hikers", laughing and joking and ccarrying heavy bags.
The fourth man was a little different from the rest, and he did not come from Beeston. Germaine Lindsay, 19, was born in Jamaica but brought up in Huddersfield. Big and strong, he was keen on martial arts and boxing but also had a gentle, playful side to his nature, according to those who knew him. But he grew more distant from them after converting to Islam and changing his name to Abdullah Shaheed Jamal. He married Samantha Lethwaite, 22, from Aylesbury in Buckinghamshire, and she converted as well. Her family did not like that, or the burqa she wore.
When Lindsay and his wife moved to Aylesbury with their year-old son this year, they kept their distance. Lindsay had driven to Luton from Aylesbury that morning, also in a hire car. He was meeting men he knew from Leeds, where he had lived for a while and worshipped at the Grand Mosque.
Within minutes of being caught on film, they would be dead. Lindsay would die on train 311 just outside Russell Square; Tanweer in the third carriage of train 204 at Aldgate and Khan in the second carriage of train 216 at Edgware Road. Hussain would die on the No 30 bus in Tavistock Square. They took a lot of people with them, and caused a lot of pain.
Should they have been stopped? Was there any way the security services could have known about what was happening? Was there a mastermind who did not die in the bombings?
The police seem to think the answer to that last question is yes. One story being told around the Met is that a fortnight earlier a Special Branch officer spotted a Pakistani who was on a "watch list" as a potential member of al-Qa'ida entering the country through a sea port. The police officer told MI5, which decided not to track the suspect because he was low risk and it could not justify the massive resources demanded by 24-hour surveilance.
So nobody knows what the man did in Britain, but he flew out from Stansted airport just before the attacks. Did he give the orders? If so, was he also in contact with other potential terrorist cells in this country? However, that is just one version of the story. The security services, stung by the idea they let someone come and go under their noses, stress that there is no evidence so far to link the four bombers with any other person.
The police were adamant, in the aftermath of the bombings, that these had been "clean skins", people with no previous criminal records known to the security services. There was just one caveat to those police statements that went almost unnoticed: one of the men had come to their attention during a previous, unspecified operation.
This may have been what the French Interior Minister, Nicolas Sarkozy, was talking about when he told journalists - in an off-guard moment - that "part of this team" had been subject to "partial arrest" in the spring of last year. Mr Sarkozy had only just been briefed by the British Home Secretary, Charles Clarke, who was livid at his comments. They were "completely and utterly untrue", he said. A Home Office statement added that none of the men had ever been arrested or detained as part of an anti-terrorist operation.
That may well be true. But The Independent on Sunday can reveal that Khan did have links - including telephone contact - with an individual who came to the attention of police and security services last year during Operation Crevice, a major anti-terrorist operation in which properties across the Home Counties were raided. A large amount of explosive material was discovered in those raids. A security source reveals that MI5 was so overstretched at the time that it had to ask for agents from its sister force, MI6, to be recalled from abroad to help keep track of suspects. Although Khan's name came up during the course of inquiries, he was not questioned. Now, a convicted member of al-Qa'ida held in American custody has said he recognises Khan.
"We are not above criticism," says a security source. "When the dust settles ... we will look to see how these people got through our net."
Balthazar Garzon, a Spanish expert in the prosecution of terrorists, has described a "second generation" of extremists who have no history of affiliation to al-Qa'ida or any other organisation, but form loose, spontaneous groups based on personal relationships. Al-Qa'ida is their inspiration, their "ideological reference point" rather than their boss, although they may seek out its help.
The London bombers were born in the country they attacked, unlike any previous terror cell known to be active in Europe. Far from hiding their identities, they had documents on them that made it clear who they were. They did not, as far as anyone knows, finance their terrorism through crime. All this suggests that they were almost mourned as victims rather than the perpetrators, their guilt underplayed.
Hussain was indeed recorded as a victim when his mother called the police casualty hotline just after 10pm on the day of the bombings. His brother, Imran, had been trying to call his mobile for ages and getting no response. A police family liaison officer visited the home in Colenso Mount to take details, as with so many of the victims. The Hussains gave the names of the "mates" their son had gone to visit London with. They were desperately worried.
When the name of Mohammed Sidique Khan was entered on the police computer, and his link with Operation Crevice came to light, detectives must have caught their breath. Was this the break they needed? The security services stress that there was a huge amount of information coming in to them after the attacks and that there was "no silver bullet".
But the phone-call theory would explain the speed with which they moved to identify the bombers and pursue anyone connected with them. Cash cards and a driving licence belonging to Hussain were found among the ruins of the No 30 bus. Documents belonging to his named friends were also discovered at the Underground bomb sites.
On Monday, police trawling through CCTV footage found him standing with the others at King's Cross. Early next morning their homes were raided, along with a local community centre where a controlled explosion was carried out. Police also raided 18 Alexandra Grove, a neat, recently built house in a leafy neighbourhood that is much better off than the down-at-heel back-to-backs of Beeston. Close to the university, it has a large student population. There are a lot of people coming and going, and visitors are much less conspicuous there than among the tightly knit communities of Beeston.
The four-bedroomed flat on the third floor of the house is owned by the Leeds Federated Housing Association. The telephone number of a man renting it is said to have been found stored in a mobile phone owned by Hussain. The flat was raided on Tuesday morning and police found several kilos of "potentially dangerous" chemicals stored in a bath. These are thought to have made the acetone peroxide, the explosive used in the London bombs, which is dangerously unstable.
Reports suggest that forensic scientists have been able to place all four of the bombers at the flat. They have also linked it with Magdi Mahmoud el-Nashar, a 33-year-old Egyptian bio-chemist who was arrested in Cairo on Friday. He had recently left Britain after completing five years study at Leeds University. The Egyptian authorities insist that he has nothing to do with al-Qa'ida. His arrest came on the same day that the Metropolitan Police Commissioner, Sir Ian Blair, mused: "What we have got to find is who encouraged them, who trained them. Who is the chemist?"
Were the bombers set up? That would explain reports of a man on the bus rummaging frantically in his bag just before the explosion. Did Hussain hear about the other explosions and panic? A neighbour who knew him said he had become a follower of Salafism, the stricter Islamic code based on practices from the earliest years of the religion. Who took advantage of his zeal for their own ends? Are there other young men like him in this country, waiting for the signal? Sir Ian seems to think so. He says there is a "very strong possibility" of further terrorist attacks.
Was Lindsay introduced to the others by a terrorist matchmaker, possibly to lead them? The others had grown up on the same streets. It was inevitable that they would run into each other at the local mosques, the youth club, or on the fields of Cross Flatts Park where Tanweer was playing cricket and football with his friends just a day before his death. These are streets where families know each other's business, and they are far more likely to talk about it among themselves than to disclose such things to strangers. Some say Khan, the oldest of the group, was a mentor to Hussain and Tanweer. Terrorism analysts would call this the role of "gatekeeper", the one who radicalised his recruits and knew how to obtain the means for an attack. However, locals say it was Hussain - the youngest but the biggest and possibly the most hot-headed of the four - who was the driving force.
At the heart of the community is the Hamara Healthy Living Centre, funded by the council. Its new building was opened in 2003 by Hilary and Tony Benn. It is only a few hundred yards from where Khan and Tanweer lived and half a mile from the home of Hussain. All of the men are said to have used the centre's facilities, although separately. Its former "youth access point" in Lodge Road has been closed for at least six months, but some locals say the bombers met there in secret, behind the graffitied steel shutters.
The key-holder, Naveed Fiaz, also worked at the Iqra bookshop selling radical Islamic material. He is currently in custody at Paddington Green police station. His home in Stratford Street was sealed off on Tuesday. On Thursday, armour-clad bomb-disposal experts sent robots in to the youth access point before carrying out a single controlled explosion. The Iqra bookshop was raided the following day and, like all the other buildings, searched for clues.
Some people in Beeston think Fiaz was the one who tipped off the police about the involvement of Khan, Hussain and Tanweer in the bombings. They suggest he is being held in custody for his own protection. This, however, is only a rumour. There are so many of those, as people struggle to understand why it was their neighbour or friend who made that journey to London with a bomb on his back. And as the police and security services try desperately - publicly and invisibly - to find out if there are any more.
The nexus of terror: how Pakistan became a world hub of extremism
Not for the first time in the war against terror, virtually every new turnin the investigation of the London bombs points to Pakistan. Every major al-Qa'ida figure in custody was captured in Pakistan. Though Pakistan's authorities deny it, senior Taliban figures are said to be living openly in the country.
Now Mohammed Junaid Babar, a Pakistani-American held in New York last year for attending a "terror summit" in Pakistan's Waziristan, has told US authorities he knows the London bomber Mohammed Sidique Khan. Another, Shahzad Tanweer, is said to have visitedreligious schools in Pakistan.
Faced with renewed global scrutiny, President Pervez Musharraf on Friday ordered police to stop banned militant groups holding meetings and thus gave a clear indication that the groups continue to enjoy public support.
President Musharraf's problem is that until 11 September 2001, Islamic militancy enjoyed official support. Pakistan took over what the Americans had started, when they backed the mujahedin fighting the Soviet occupation of Afghanistan.
Once the US lost interest in Afghanistan, Pakistan adopted the networks of militants. It used them to fight the Indians in Kashmir, and to install the Taliban regime in Afghanistan.
But September 2001 made President Musharraf rethink his strategy. He managed a spectacular U-turn, siding with the US against the Taliban at great personal risk.
Public support, however, remains strong and factions in the military and the ISI, Pakistan's intelligence service, retain links to militants. Some believe Bin Laden has evaded capture only with assistance from elements within the ISI.