Three of the July 7 suicide bombers visited London for a dummy run nine days before carrying out the atrocity, police said today.
Detectives said today that the bombers, two of whom were carrying rucksacks, scouted the Underground system for three hours on June 28.
CCTV pictures, found among 80,000 tapes collected by police, show Mohammed Sidique Khan, the cell's presumed ringleader, with Shahzad Tanweer and Jermaine Lindsay travelling to London during the morning rush hour following the same route they used on July 7.
On June 28 Khan met Tanweer and Lindsay at Luton train station at around 8:10am. Wearing casual clothes and carrying only two light rucksacks between them, the three men took a Thameslink train and arrived at King’s Cross at approximately 8:55am.
The three men then apparently travelled back and forth between King's Cross and Baker Street Underground stations for three hours, appearing at Baker Street at around noon before heading back to King's Cross and returning to Luton, arriving at 1:40pm.
"The obvious suggestion is they possibly were conducting a reconnaissance on that day. We know that’s part of terrorist methodology," said Peter Clarke, head of Scotland Yard's anti-terrorist branch. "It was a dry run."
The reconnaissance mission, along with the videotape of one of the bombers that was broadcast on al-Jazeera earlier this month, suggests that the bombings were carefully orchestrated and that the bombers knew they were going to die in the attacks.
The trio who travelled to London on June 28 blew themselves up within seconds of each other on Underground trains during the rush hour on July 7.
The fourth bomber, Hasib Hussain, set off his rucksack bomb on a No 30 bus about an hour later. It is believed that Hussain, who did not take part in the dry run, intended to bomb a Northern Line train travelling north from Euston but found the service suspended on the morning of July 7. A total of 52 commuters were killed in the attacks, as well as the four bombers.
Detectives were prompted to study large amounts of CCTV footage from June 28 after they found receipts and ticket stubs from the day during their searches of the homes of the bombers in West Yorkshire.
Scotland Yard is still investigating why the July 7 bombers left a collection of bombs and nailbombs in a car at Luton train station if they did not have accomplices who could have used them.
Mr Clarke said that there was as yet "no indication" to suggest there were supposed to be more than the four suicide attackers on the day of the bombings.
It was also revealed today that the bombers used a peroxide-based explosive to carry out the bombings. Two bottles of peroxide explosive encased in nails were found in a bag under the front passenger seat of a red Nissan Micra which three of the bombers had used to travel to Luton on the day of the attacks.
Mr Clarke said it would be speculation to suggest that two other bombers were supposed to turn up to join the suicide mission that day. "We found two viable devices ready to go underneath the front passenger seat," he said. "We do not know what these were intended for. To speculate is interesting but unhelpful - but of course it is of real concern that there were more explosives in the possession of these people on that day."
Today's briefing gave another glimpse of the enormous scale of the police investigation into the July 7 attacks and the failed bombings of July 21. Detectives have now taken more than 3,000 witness statements and gathered more than 30,000 pieces of evidence in the inquiry.
The police said today that more than 1,000 of those exhibits were taken from a single flat in Alexander Grove, Leeds, where the explosives used on July 7 were made.
Among the 15 sites detectives have searched is a landfill in West Yorkshire, where examinations are still going on, Mr Clarke said. The Skelton Grange landfill site contains the equivalent volume of 18 Olympic swimming pools but police think material from a bomb factory found at Alexander Grove, in Leeds, may have found its way there.
News of the bombers' dummy run came hours after al-Qaeda made its first explicit claim of responsibility for the London bombings.
In a tape broadcast on al-Jazeera last night, Ayman al-Zawahri, second in command of the terror group, said that it had been "honoured to launch" the "blessed" London attacks.
In an earlier video message broadcast on September 2, which also featured Mohammad Sidique Khan, al-Zawahri described the attacks as a "slap in the face" to the policies of Tony Blair but did not directly say that al-Qaeda had carried them out.
But in the latest video, which carried the logo of al-Sahab Media, an organisation that has produced other al-Qaeda propoganda, al-Zawahri said the bombings were a response to British aggression against the Islamic world.
"The blessed London attack was one which al-Qaeda was honoured to launch against the British Crusader’s arrogance and against the American Crusader aggression on the Islamic nation for 100 years," said al-Zawahri.
Chronology of the two days
June 28
Sidique Khan and Tanweer meet Lindsay at Luton station around 8.10am. The three men buy tickets and catch a train to King's Cross.
The men arrive at King's Cross at 8.55am and are also seen at Baker Street at midday.
The bombers leave King's Cross at 1250 BST and arrive back in Luton at 1340 BST.
July 7
Khan, Tanweer and Hussain leave Leeds at 4am in a silver Nissan Micra.
Lindsay arrives at Luton station at 5am and the other three join him at 6.51am.
All four take a 7:48am Thameslink train to King's Cross, arriving around 8:30am before dispersing.
Khan and Tanweer took Circle Line trains, heading east and west respectively, while Lindsay boarded a southbound Piccadilly Line train. Hussain, it is believed, intended to bomb a Northern Line train but found the service suspended.
Khan, Tanweer and Lindsay all set off their bombs at 8:50am while Hussain, panicked, was evacuated from King's Cross station with thousands of other commuters. After making a series of frantic phone calls, Hussain boarded a No 30 bus and blew it up at 9.47am, killing 13 people.
