The July 7 gang were quite deliberate about what they left behind. They made sure that identity documents belonging to them would be found after their deaths so they could be identified as “martyrs”. Did they leave the explosives — nail bombs and solid blocks of the peroxide mixture — so the authorities would know that British extremists were capable of making such devices, or was there another member?

One theory is that the nail bombs were to be thrown at police if the bombers were intercepted. The reports say there are no indications that there was a fifth bomber, but investigations continue. And why have the keys for the blue Nissan Micra never been found?

Who passed Mohammad Sidique Khan’s video will to the al-Qaeda leadership?

Khan, the leader of the bomb cell, made a video testament in which he declared himself to be a soldier who carried out the bombings to avenge the atrocities against his Muslim brothers and sisters.

An Arabic news channel received it from sources who passed al-Qaeda material to its journalists. Khan’s video was edited with a message from Ayman al-Zawahiri, al-Qaeda’s second-in-command, who praised the bombings and claimed them on behalf of Osama bin Laden.

Khan also left a written will modelled on a document left by a British-born al-Qaeda guerrilla who had died in the Tora Bora mountains in Afghanistan.

Who masterminded the operation?

The official reports say media claims that a mastermind left Britain days before the attacks are inaccurate. But they state that a known extremist did leave Britain just before

July 7 and that Khan and his gang must have had advice from experienced terrorists.

The Intelligence and Security Committee says the assessment now is that Khan and Shehzad Tanweer “had some contact with al-Qaeda figures” in Pakistan in 2004. They are believed to have had “operational training” and “may have had advice or direction from individuals there”.

The Home Office narrative states that the bombers were in touch with individuals in Pakistan between April and July 2005.

In June 2005, Khan and Tanweer went on a whitewater rafting trip in North Wales. Witnesses report that there was a burly man in the party who spoke only Urdu and appeared to hold sway. Khan took orders from him. The authorities have not traced this man.

Did the gang have help from a bomb maker?

The ingredients for the July 7 bombs were easily obtainable and instructions for making the mixture are widely available on Islamist internet sites. But the explosives were not easily assembled.

The peroxide-based mixture was volatile and prone to deterioration if not kept cold. It gave off toxic fumes, lightening the colour of Khan’s and Tanweer’s hair and killing the flowers in the windowboxes of the Leeds flat that they used as a bomb factory.

The Home Office report states that it is “likely that the group would have had advice from someone with previous experience, given the careful handling required to ensure safety during the bombmaking process”.

Pakistani sources suggest that Khan had extensive explosives training in camps linked to al-Qaeda.

Magdy al-Nashar, an Egyptian biochemist who lived in Leeds and had links with members of the bomb cell, has never returned from Cairo since going there last summer. British police would like to speak to him.

Why was Hasib Hussain not on the bombers’ reconaissance mission and why did he not detonate his bomb on the Tube?

Hussain, 18, may have been a last-minute recruit. When Khan, Tanweer and Jermaine Lindsay visited London on June 28 and followed the route they would take on July 7, he did not accompany them. While the other three killed themselves and other passengers on the Tube, Hussain detonated his device almost an hour later on a bus, after buying a 9v battery. Had he taken fright or been unable to get a train because of problems on the Northern Line? Or did his detonator fail, forcing him to buy a new battery?

Why July 7?

The reports discount the idea that it was selected to coincide with the G8 summit in Scotland. It was also the day after London was selected to host the 2012 Olympics. The radical cleric Abu Hamza al-Masri was due to stand trial in London on July 7. His case was postponed because of the attacks.

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