By Chris Summers and Dominic Casciani
The bombs that failed to explode in London on 21 July 2005 were almost identical to the ones that killed 52 people on the transport network two weeks earlier. But why didn't they go off?
Investigators spent many hours examining the devices used on 21 July and comparing them with the 7 July bombs.
There was only one minor difference - the 7/7 bombers mixed ground pepper into the mixture while the gang two weeks later used chapatti flour.
But Dr Stuart Black, an explosives expert who gave evidence at the trial, said that was not the reason the devices failed to explode.
The main charge - a mixture of hydrogen peroxide and chapatti flour - was packed into a plastic container stuffed inside a rucksack.
The would-be bombers - who have been found guilty at Woolwich Crown Court of conspiracy to murder - taped bolts and washers to the outside of the container to act as shrapnel.
The main charge would be set off by a detonator inside a short tube fitted onto a modified light bulb.
To trigger the blast, the bombers simply touched together a battery and a terminal both of which they could hold inconspicuously in their hands.
The electric current passed down two wires, through a slit in their rucksacks and into the modified bulb. The current in the bulb was enough to trigger the detonator - but the main charge did not explode. The question remains why.
Two theories were offered at the trial at Woolwich Crown Court.
The plot's prime mover, Muktar Ibrahim, himself suggested the device would not explode because he had deliberately diluted it with tap water. This was part of his plan, he claimed, to construct bombs that were as realistic as possible, but unable to explode, as part of his "demonstration" against the Iraq war.
But the prosecution offered another view.
Clifford Todd, the chief investigator with the government's Forensic Explosives Laboratory, spent months working out how the bombs on 7/7 and 21/7 were designed.
No-one had ever come across devices with these characteristics before. But when Hussein Osman first claimed that it had been a hoax, it was Mr Todd's job to separate the science from the science fiction.
Hydrogen peroxide is well known among experts as a potential bomb ingredient- but only if used in the correct concentration.
Clifford Todd: Spent months studying the devices
The trial heard that Ibrahim and Yassin Omar spent many hours heating the hair bleach in the New Southgate bomb factory to achieve that concentration - and it is not clear if they succeeded.
But when it came to detonation, the hydrogen peroxide failed to react.
All four bombs simply made a "popping noise" and began leaking onto the floor of the three Tube trains and one bus where they were found.
Clifford Todd's team of scientists took small samples for chemical analysis - but when some of the mixture continued bubbling, experts were forced to destroy the rest amid fears of an explosion.
The scientists at the FEL realised they would need to construct copies of the devices in order to test the hoax theory further - and it took months of carefully planning by a large team to come up with a safe way of trying to do what the bombers did in a council flat.
Failure: But impossible to predict how it would work
When it came to detonating the device, the situation was so dangerous that the scientists relied on a remote-controlled robotic device to insert the detonator and initiate the explosion. The device worked.
More importantly, Mr Todd's team established there was no way in the world that Muktar Ibrahim could have known how the devices would have behaved on the day - his claims of a hoax were lies.
Dr Black is in no doubt that the 21 July devices were potentially lethal.
Dr Black said: "Hydrogen peroxide is widely used in explosive devices in Iraq and elsewhere but July 2005 was the first time it was used in the UK. It was definitely a turning point."
"It would have been devastating. The death toll may not have been as high as 7/7 but that is only because the trains were less crowded. The devastation would have been the same. It was very destructive."
Back in the laboratory, Clifford Todd's team had one last check to make: was it possible that Ibrahim had learnt how to build these bombs from precise instructions in academic, military or scientific journals?
Their research drew a blank - with the team finding no mention at all of this type of device.
Ibrahim himself claimed he had downloaded the instructions from the internet - although in the mass of other documentary evidence nothing was found at his home.
Another theory is that he learnt the skills in a jihadi training camp over the winter of 2004 somewhere in Pakistan.
However he gained the skills, the bombs costs very little to construct - the main cost of £550 being for the hydrogen peroxide.