The bombs used on 7 July were made from peroxide-based explosives and showed sophistication and expertise, said the police. They were kept in "cooler" boxes prior to use. It was also disclosed that a landfill site at Skelton Grange, in west Yorkshire, the "size of 18 Olympic swimming pools" was being searched for material which may have been dumped from the group's "bomb factory" at Alexander Grove in Leeds.
The pictures were tracked down after police studied thousands of hours of CCTV footage filmed in the days leading up to the bombings. The new images show Mohammad Sidique Khan, Shahzad Tanweer and Germaine Lindsay at Luton, King's Cross and Baker Street stations on 28 June. Significantly, the fourth bomber, Hasib Hussain, who blew himself up on the No 30 bus, is missing from the footage.
Investigators also want to find out if the rest of the bombing team met another person during their "dry run" visit to the capital.
Police have discovered that, on the day of the bombings, Khan, Tanweer and Hussain travelled down from Leeds in the Nissan Micra, leaving the city at 4am. They arrived at Luton station at 6.51am, where they were met by Lindsay, who had been waiting for them since 5am. The four then went into London on a Thameslink train.
Inquiries have also led the police to conclude that Hussain may have initially boarded a different bus before getting on the No 30 to Tavistock Square in central London, where he exploded his device.
On 28 June, Khan and Tanweer travelled down from their homes in Leeds, met Lindsay at 8.10am at Luton station and arrived at King's Cross at 8.55am. Later in the morning they were captured on CCTV at Baker Street and then departed from King's Cross at 12.50pm before arriving back at Luton at 1.40pm. They were dressed in T-shirts, casual trousers and trainers, and two of them were carrying rucksacks.
Referring to the "dry run", Deputy Assistant Commissioner Peter Clarke, the head of the Anti-Terrorist Branch, said: "The implication is that they were possibly conducting reconnaissance on that day. We know that is part of a terrorist's methodology: to check timings, layout and security precautions.
"We are trying to reconstruct their movements as far as we can on that day. What we want to know is where else they went and whether they met anyone else while they were in London.We want to find out who supported them, who encouraged them, who knew what they were doing and we want to explore the international connections as well."
Mr Clarke added that, on the day of the bombing, "We found two viable devices ready to go underneath the front passenger seat [of the Nissan Micra]. It is, of course, of real concern that there were more explosives in the possession of these people on that day.
"The devices used on 7 July were effectively made and whoever had manufactured them had done some good research and had been well trained."
Mr Clarke also said that the investigation was attempting to trace the movements of Khan and Tanweer during visits to Pakistan but that the process of analysing their trips was "long and complicated" and the men had legitimate family links to the area.
He said officers had not uncovered suicide video messages from any of the bombers, other than that from Sidique Khan. The Khan video, which was broadcast on an Arab television station earlier this month, showed the 30-year-old blaming the British public for the July 7 attacks.
"We're looking at this very closely," Mr Clarke said. "We want to identify when the Khan video was made and where it was made. There is a lot of work going on to find that out."
Forensic experts have found traces of HMTP, a type of peroxide explosive, and recovered 14 bits of material, some of which appeared to be component parts for other bombs. Detectives believe the bombs were made at the Alexander Grove flat. The search of the premises took six weeks and involved the removal of 2,000 exhibits.
So far, more than 3,000 witness statements have been taken, 30,000 items are being treated as exhibits and there have been searches at 15 different locations.
Andy Hayman, Assistant Commissioner for Specialist Operations at the Metropolitan Police, described the investigation as " a colossal undertaking". He added: "It is one of the most complex and wide-ranging that we have faced, but it continues to make progress and we are confident our continued efforts will bring further results."
The questions still to be answered
Why was Hasib Hussein, the man who carried out the suicide bombing on the No 30 bus in Tavistock Square on 7 July, not part of the "dry run" on 28 June? Was he a late recruit to the July 7 cell?
What was Hussein's original target? He is believed to have attempted to board a northbound Northern line train but failed to do so due to disruption to the service at King's Cross. He later boarded the No 30 bus.
Why were two primed devices found in a rucksack in the bombers' car left at Luton station on 7 July? Were they due to be collected by a second cell?
Were the four bombers acting alone? Did they meet a "handler" or "mastermind" in London on either 28 June or 7 July?
Who made the bombs? Police say the devices showed sophistication and expertise, yet none of the bombers is known to have had any detailed knowledge of explosives.
Was al-Qa'ida behind the July 7 bombs? Earlier this week Osama bin Laden's deputy Ayman al-Zawahri claimed responsibility for the attacks although investigators have yet to find any irrefutable connection with the international terror network.
What was the Pakistan connection? At least two of the bombers travelled to Pakistan several months before the bomb attacks.Reuse content