The security services were last night coming to terms with the nightmare scenario that the London bombers were ordinary citizens who had never come to the attention of the authorities.
A senior security source said the existence of so-called "clean skins" made the job of preventing further attacks even more difficult.
"How many clean skins have we got waiting in the wings?" he asked.
"What we don't know is whether someone came in under the al Qaida methodology, whether they came in, did the preparation and left the country the day before the attacks."
Sources said there was "strong" forensic evidence linking 22-year-old Shahzad Tanweer to the blast on the underground train near Aldgate.
His semi-detached home in the Beeston area of Leeds, where he had lived all his life, was sealed off yesterday.
The driving licence and cash cards of another suspect, who was 19 and from Leeds, were found in the mangled wreckage of the number 30 bus which blew up in Tavistock Square, killing 13 people.
He had told his parents he was going to London with friends on the day of the attacks and at 10.20pm that day they reported him missing to the police casualty bureau.
Documents belonging to a third suspect, who was 30 and from Dewsbury, West Yorkshire, were found in the debris of the Edgware Road blast. All three men were believed to have been friends.
They are thought to have used hire cars to travel from West Yorkshire to Luton last Thursday morning.
All four men then boarded a Thameslink rail service to King's Cross where they were captured on CCTV just before 8.30am.
They then split up, three of them detonating their bombs on separate trains simultaneously at 8.50am.
The bus bomb detonated 57 minutes later. Detectives are still unsure whether that bomb had been destined for a fourth tube train.
One theory is that three bombers travelled west, east and south on the London
Underground from King's Cross and the fourth was supposed to go north but the Northern Line was closed that day.
Police were last night also examining a second car recovered in the Luton area which is thought to have been used by the bombers.
As police try to formally identify the bombers, some bodies from the blast sites are being fast tracked through the identification process. The first victim to be named was mother-of-two Susan Levy, aged 53, from Newgate Street Village, Cuffley, Herts.
An inquest into her death has been opened and adjourned at St Pancras Coroner's Court in London by Dr Andrew Reid.
Mrs Levy, mother of Daniel, 25 and James, 23, died in the Underground blast at King's Cross.
Jamie Gordon, 30, from Enfield, and Philip Russell, 28, from west London, were killed in the Tavistock Square bus explosion.
Mr Gordon, a City worker, was described by his family and partner Yvonne Nash as a "kind, caring person who always put other people first".
Mr Russell took his fateful journey on the number 30 bus after being evacuated from Euston tube station.