Newly released CCTV footage shows the 7 July London bombers staged a practice run nine days before the attack.
Detectives reconstructed the bombers' movements after studying thousands of hours of film as part of the probe into the blasts which killed 52 people.
CCTV images show three of the bombers entering Luton station, before travelling to King's Cross station where they are also pictured.
Officers are keen to find out if the men met anyone else on the day.
The three, Mohammad Sidique Khan, Shehzad Tanweer and Germaine Lindsay, were conducting a carefully planned reconnaissance exercise, police said.
Deputy Assistant Commissioner Peter Clarke, head of Scotland Yard's Anti-Terrorist Branch, said: "What we want to know is where else they went and did they meet anybody else while they were in London?
"If any member of the public thinks that they know something about the movement of these men on that day, they should call us on the anti-terrorist hotline."
He added that it was "part of a terrorist's methodology" to check timings, lay-out and security precautions.
Police traced the movements after recovering tickets and receipts from houses connected to the bombers which pointed to their trip.
Mr Clarke said the investigation would carry on for months. More than 3,000 plus witness statements had been gathered and 80,000 CCTV tapes analysed.
Police revealed that two bombs were found in a car left by the attackers at Luton train station on 7 July.
It has also emerged that a landfill site in Skelton Grange, West Yorkshire, is being searched in a bid to uncover more clues.
DUMMY RUN DETAILS
Sidique Khan and Tanweer meet Lindsay at Luton station around 0810 BST
The trio buy tickets and catch a train to King's Cross
The men arrive at King's Cross at 0855 BST 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
A dozen officers in fluorescent jackets could be seen on Tuesday afternoon working on the site.
The men, who were wearing overalls under their jackets and white safety hats, appeared to be systematically searching a small area of the rubbish heap helped by two mechanical diggers.
One local resident said: "They've been here for weeks, dozens of them.
"They've been searching the same bit of rubbish every day it seems.
"The diggers skim off a layer at a time and then they move in and search it quite painstakingly."
A landfill site the size of 18 Olympic swimming pools is being searched
Detectives believe the site could be connected to the apparent "bomb factory" at a flat in Alexander Grove in Leeds.
Meanwhile, al-Qaeda has said for the first time the group carried out the attacks.
In a videotaped message aired on Arab television station al-Jazeera, al-Qaeda deputy leader Ayman al-Zawahri said the group had the "honour" of carrying out the attacks.
The 7 July bomb attacks killed 56 people - including the four bombers - and injured more than 700.
Three bombs were detonated on underground trains just outside Liverpool Street and Edgware Road stations, and on another travelling between King's Cross and Russell Square.
The fourth explosion took place on a double-decker bus in Tavistock Square, not far from King's Cross.
Evidence of a reconnaissance mission supports the theory that all four had planned to detonate their rucksack bombs on the Underground system.
It is believed that the bus bomber, Hasib Hussain, was prevented from getting onto the Northern Line on the day of the attacks because the service had been disrupted.
The other bombers - Tanweer, Lindsay and Sidique Khan - detonated their devices almost simultaneously.
Anyone with information in connection with the London bombings should ring the anti-terrorist hotline on 0800789321.