The tall visitor from Pakistan spoke no English, and refused to be photographed like the other men on the adventure training trip to Wales a month before the bombings.

Staff at the adventure centre described how Mohammad Sidique Khan, the oldest of the suicide bombers, translated instructions to their Pakistani guest, who exuded considerable influence over the rest of the group. Most of his remarks were addressed to Khan, 30, and his fellow bomber, Shehzad Tanweer, who was 22.

Locals from their Yorkshire neighbourhoods remember seeing the same figure at the Hamara Youth Centre, where Khan put his followers through martial arts training. “Then, as quickly as the man appeared on the scene, he vanished from the community just before the attacks,” one police source said.

Where he went and why he was with the bombers is one of the key questions still confounding those leading Britain’s biggest murder hunt.

The elusive Pakistani is among a number of people that they are desperate to trace, including at least two local associates of the bombers who have not been seen since the attacks, which killed 56 people.

Senior officers insist they are making progress, but still nobody has been charged and they appear no nearer to finding other members of the cell.

Police have never believed that the four British-born bombers were acting alone and wonder if the mystery Pakistani man was sent to help the group to finalise their plans.

Three weeks after the rafting expedition, three of the bombers — Khan, Tanweer and Jermaine Lindsay — went on a reconnaissance trip to London. Their guest from Pakistan disappeared shortly after this June 28 visit.

Kate Blyth, one of the instructors at the National White- water Rafting Centre, near Bala, remembers seeing the man on June 4. She told how 12 men, all of Asian origin, turned up together in two cars. Staff noticed how Khan was telling the others what to do, except that his manner changed when he addressed the Pakistani guest. Ms Blyth described the group’s loutish behaviour as she attempted to give them a safety briefing, saying that Khan was the worst offender.

Yet the mystery Pakistani visitor was careful to shield his face from the official photographer. Police are trying to discover where the man stayed during his time in Yorkshire and whether he met Khan, Tanweer and the youngest of the bombers, Hasib Hussain, during their trips to Pakistan in the weeks leading up to the attacks.

Officers have spent weeks in Lahore and Karachi but have failed to establish where the bombers went on what was supposed to be a holiday to visit family.

Saudi Arabia, Egypt and Pakistan have publically declared their full co-operation but British security officials have been frustrated at being refused access to key witnesses and the lack of success at tracing those who may have inspired or organised the London attacks.

Khan’s video testament, the typical suicide bomber’s propaganda farewell, ended speculation that the men had been duped into becoming martyrs, though there are still sceptics among the Muslim community in Yorkshire.

UNANSWERED QUESTIONS

1. Who first recruited the four British-born bombers?

2. When were they told that this was a suicide mission, and what the target was?

3. Who built the bombs?

4. Why were so many explosives and primed bombs left in the car at Luton station car park, and where are the keys to that vehicle? 5. Why did Jermaine Lindsay have a gun in his car?

6. What are the links between the 7/7 and 21/7 bombers?

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