After having second thoughts, Hussain then took refuge at McDonald’s, in King’s Cross, and tried to contact his fellow bombers to find out if they too had failed to go through with their suicide and murder pact.

But Hussain, the youngest of the bombers, aged 18, received no replies. At the same time, he would have seen the first of the hundreds of commuters streaming out of King’s Cross as the system ground to a halt when the three other bombs detonated.

Realising then that he was completely alone, he steeled himself to go through with his attack after all. There was by then no possibility of boarding any trains, so Hussain got onto a diverted bus packed with commuters and finally detonated his bomb.

The explosion on the number 30 bus in Tavistock Square killed 13 people.

The theory emerged last night after it was revealed that, contrary to earlier reports, there were no problems on the north-bound section of the Northern Line.

Police believe that Hussain has intended to attack a Northern Line train to complete his part of the suicide pact that would have sketched out a ‘burning cross’ of bombs in the heart of London.

The 18-year-old’s moments of crisis are believed to have been captured on CCTV pictures at the burger restaurant in King’s Cross and will help detectives to piece together the ‘missing hour’ between Hussain’s failure to board the tube train and his blowing up the number 30 bus.

The youngest of the July 7 bombers, he made three desperate telephone calls begging for help from the other members of the terror cell minutes before he blew himself up on a London bus.

The frantic last messages are seen by Scotland Yard as vivid proof that the British-born Muslim extremists intended to die in the attacks.

Knowing that all four men were supposed to synchronise the timing of the explosions, Hussain ran out of King’s Cross Underground station and tried to reach his accomplices by mobile telephone.

It was just before 9am, but by then all his fellow bombers were already dead. The other three had triggered their devices within seconds of one another at 8.50am.

Hussain is believed to have first called Mohammad Sidique Khan, 30, the alleged leader of the group, saying: “I can’t get on a train. What should I do ?” Then in quick succession he left the same message for Shehzad Tanweer and Jermaine Lindsay as, clearly agitated about his next move, he hurried away from the station.

A police source who has heard the telephone calls said: “His voice was getting more and more frantic with each call.” Investigators could tell from his breathless voice that Hussain was walking fast as he made these calls.

After his stop at McDonald’s Hussain climbed on to a passing No 30 double-decker bus in Tavistock Square.

Survivors remember him looking flustered as he rummaged inside the large black rucksack on his lap.

Seconds later he detonated the homemade explosive, killing 13 people. It was 9.47am — 57 minutes after his accomplices had struck.

Forensic science experts who have examined the remains of the bombs are now certain that they were all triggered manually, which proves that the men were not duped into becoming suicide bombers.

One source close to the investigation said: “He must have known from the fact that he couldn’t raise any of his accomplices that they were already dead, and yet he picked a new target and went through with his plan. We need to know who persuaded him to behave like that”.

At no stage did he try to telephone anyone from his family.

The Yard has failed to find any of the support network who helped the 7/7 bombers though they accept that the men’s families knew nothing of their plans.

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