Three of the men responsible for bombing London on 7 July did a practice journey from Luton, nine days before carrying out their plans for real.
Police trawled through thousands of hours of CCTV film and found images of three of the bombers doing a trial run of their final journey on 28 June.
CCTV images show the men entering Luton station at 8.10, before getting off the train in Kings Cross, also on film.
Detectives say the dummy run proves the bombing were well planned.
They believe bombers Shehzad Tanweer, Germaine Lindsay and Mohammad Sidique Khan checked timings, layout and security at stations during their journey.
Detectives are now trying to find out if the men met up with anyone else during their London visit.
Al-Qaeda 'claim bombings'
The bombs on 7 July killed 56 people - including the four bombers - and injured more than 700.
During the attacks, the men exploded three bombs on the tube lines just before 9am.
It's thought the bus bomber, Hasib Hussain, was stopped from getting onto the Northern Line because the service had been disrupted. His bomb exploded on the number 30 bus in Tavistock Square, near Kings Cross.
In a videotaped message shown recently on the Arab television station al-Jazeera, al-Qaeda deputy leader Ayman al-Zawahri said the group had the "honour" of carrying out the attacks.