The terrorist threat facing Britain assumed a chilling new dimension last night as speculation mounted that the four men police believe responsible for last Thursday's bomb attacks in London may have been on a "suicide" mission.
All four suspects - British nationals, three of them from west Yorkshire - were captured on CCTV at King's Cross station, carrying rucksacks, shortly before 8.30am on the day of the attacks, which have so far claimed the lives of at least 52 people.
One of the suspects was reported missing to the Central Casualty Bureau shortly after 10am on Thursday and some of his belongings were subsequently found amid the wreckage of the Number 30 bus which exploded in Tavistock Square.
With major police operations continuing in Leeds and Luton - and a relative of one of the suspects having been arrested in Yorkshire and brought back to London for questioning by counter terror chiefs - Scotland Yard said they believed at least one of the suspected bombers died in Thursday's blasts.
Forensic evidence led them to believe that the bomber responsible for the train explosion at Aldgate had died there. The discovery of belongings identifying two of the other suspects led police sources to say it was likely that at least three had died.
However, uncertainty remained last might about the fourth suspect. Police were unable to say whether he might have fled or if his remains might still be buried beneath King's Cross.
With the investigation into the London bombings now moving "at great speed", a day of dramatic developments saw over 600 people evacuated as police raided six homes in Leeds and seized a car in a carpark at Luton railway station in which they found explosives.
The men are reported to have boarded a Thameslink train from there to King's Cross, although it is unclear how they travelled to Luton.
The car in Luton was subject to three controlled explosions, as police said discoveries in Leeds - later described as "a significant amount" of explosives material - had prompted similar police actions to ensure no risk to the public.
Security experts reflected immediate relief that, if correct, the police assessment meant the terror cell responsible for Thursday's bombings might not still be at large and on the verge of another strike. However, that was tempered by the fear that "home-grown" suicide bombers operating in Britain for the first time would significantly raise the level of the threat.
British prime minister Tony Blair will hold urgent talks with Britain's Muslim leaders amid heightened fears about a possible "backlash" against the Muslim community.
The latest developments in the inquiry, and their possible implications, are likely to overshadow Prime Minister's Questions in the Commons this afternoon.
At the conclusion of yesterday's press conference, assistant commissioner Andy Hayman said the bombings had been carried out "by extremists and criminals", on which basis "no one should attempt to smear any community with these acts".
Deputy assistant commissioner Peter Clarke, head of the anti-terrorist branch, said the police investigation was "moving at great speed" and that it had led them "quite early to have concerns" about the movements and activities of the four men.
They were trying to establish their movements in the run-up to the attacks and to establish if they all died in the explosions. Six warrants had been executed in the west Yorkshire area under the Terrorism Act, including the home addresses of the three men referred to.
He said the detailed forensic examination which would follow would take "some time" to complete. The family of one of the men had reported him missing, and the police now knew that he had been joined on his journey to London by three other men.