By Dominic Casciani
BBC News Home affairs
The judge in the trial of the 21 July bombers has said he has "no doubt" their failed plot and the deadly London attacks two weeks earlier were both inspired and controlled by al-Qaeda.
Mohammed Sidique Khan and Muktar Ibrahim: Did they meet?
On Thursday 7 July 2005, Mohammed Sidique Khan led three other suicide bombers onto the Tube and a bus and killed 52 commuters.
Mukhtar Ibrahim's planned attacks came exactly two weeks later.
But what's the evidence of a genuine link?
During the trial, both the prosecution and the defendants themselves brought up incidents that pointed towards the men having gone through a journey of radicalisation resembling the journeys of other bombers.
Critically, some of the men took notice of speeches and appearances by one of the then key propagandists in the UK, the now-jailed preacher Abu Hamza al-Masri. Both Ibrahim and Yassin Omar visited Finsbury Park Mosque, then under Abu Hamza's control.
Khan: Nov 04 - Feb 05
Ibrahim: Dec 04 - Mar 05
Bomb preparations began in earnest after trips
At the bomb factory in north-west London, the home of Yassin Omar, police found so-called "jihadi" videos. These films included footage of attacks such as 9/11 and terrible scenes of the ritualistic murders of hostages.
Police also found recordings of pseudo-theological justifications for suicide bombings. Yassin Omar would in February 2005 argue with a local mosque imam who told a congregation that suicide bombing was contrary to the peaceful teachings of Islam.
But the most important clue may be the bombs themselves.
All of the bombs on 7 July and 21 July were based around the use of hydrogen peroxide hair bleach.
Government scientists have been coming across hydrogen peroxide-based explosives since the mid-1990s.
But the devices used on 21 July and 7 July had never been seen before.
Bombs: Scientifically similar, details different
The scientists who studied the Leeds bomb factory linked to the 7/7 attacks had to start from scratch in attempting to understand the processes used by the men to create the explosives.
The BBC understands the 7/7 and 21/7 bombs were similar in scientific principal but differed in detail. Both needed organic material to explode. The 7/7 bombs used black pepper and the 21/7 bombs used chapatti flour.
Forensic scientists have found no record in official, scientific or academic literature of explosives professionals ever building these kinds of bombs: they were just too dangerous and unpredictable to handle.
Secondly, police officers do not believe the internet would have provided the bombers with all the technical know-how to build viable devices - manuals on suspect websites will only take you so far, they say.
That leaves two possibilities. Firstly, that Khan and Ibrahim independently but coincidentally came up with devices that were almost identical, despite having no background in science or chemistry.
Or, secondly, they were taught by someone. And if they were taught, was it the same person - and was it at the same time?
On 11 December 2004 Ibrahim headed to Pakistan along with two other London men, Shakeel Ismail and Razwan Majid.
They were carrying large amounts of cash, cold-weather gear and pages of a first-aid manual on ballistic injuries. They were stopped and interviewed at Heathrow and then allowed on their way as they had not committed any crime.
However, the equipment they were carrying was consistent with previous cases of British men heading off for jihadi training camps, or even to join Mujihadeen fighters in Kashmir or Afghanistan.
Of the three, only Ibrahim came home in March 2005. Ismail and Majid are still missing. Woolwich Crown Court heard suggestions that the men were dead. Ibrahim told the trial they were probably studying Islam.
The dates of Ibrahim's trip roughly coincide with a stay in Pakistan by Mohammed Sidique Khan and Shehzad Tanweer, one of the other 7 July bombers. Khan and Tanweer left for Pakistan on 19 November 2004 and returned on 8 February 2005.
In both cases, on their return to the UK planning began in earnest for the bombings.