The 7 July London bombings inquests have heard how counter-terrorism police spotted the attackers on CCTV.
The four suspects were isolated on railway station footage within four days of the attacks.
Day three of the inquests saw previously unreleased CCTV of the men buying DIY supplies for the bombs.
The inquests into the 52 deaths in 2005 will take months to set out the precise movements of the men and how the victims died.
One of the first priorities for Scotland Yard's Counter Terrorism Command was to recover as much CCTV footage as possible from anywhere that officers suspected was connected with the attacks.
Detective Inspector Ewan Kindness was in charge of the 100-strong CCTV recovery team at the Metropolitan Police. They worked backwards from the bomb sites to try to identify the attackers' movements.
On the fourth day of reviewing footage, one officer isolated the suspects on footage obtained from 76 cameras at Kings Cross station.
Det Insp Kindness said: "The officer engaged in CCTV recovery was ex-military. He saw four individuals walking through. They were walking two by two and he thought it was significant.
"They were carrying large rucksacks. He brought it to my attention and I concurred with him that it was a priority for us."
Officers then compared the footage to photo driving licences belonging to ringleader Mohammad Sidique Khan and Hasib Hussain, recovered from the the bomb sites.
Mr Kindness said: "Although it was grainy, we were convinced at that stage that they were the same individuals."
DI Kindness said officers were able to retrace the steps of the bombers to the Thameslink platform at Kings Cross - and then back to Luton station where the four had met up.
The detective told Lady Justice Hallett, the coroner, that the CCTV operation was hampered because some organisations did not hold footage for long enough - or it was unobtainable.
London Underground had a policy of only holding footage for seven days. There was also no footage from the Number 30 bus attacked at Tavistock Square because its hard drive had been corrupted.
By July 12, police had established the bombers had arrived at Luton in two cars, a Nissan Micra and a red Fiat Brava, which was later towed away.
The inquests also saw footage of the bombers buying parts for their bombs in a B&Q hardware store and Asda supermarket.
Khan, Hussain and another of the bombers, Shehzad Tanweer, visited the DIY store three days before the attacks and bought a set of pliers and lightbulbs which were destined to be an essential element of the bombs.
The day before the bombings, Khan and Tanweer bought bags of ice from Asda. The ice was essential to ensure the bombs remained stable as they travelled to London. A receipt for the 15 bags of ice was recovered at the site of Khan's Edgware Road bomb.
The inquests continue.