Police have appealed for anyone who may have seen the Tavistock Square bus bomber as he waited an hour before setting off his explosives.
Hasib Hussain did not trigger his explosives until 9.47am last Thursday - an hour after three other suicide attackers set off their bombs on three Tube trains.
Police released CCTV pictures of Hussain carrying a large rucksack at Luton station at 7.20am, on the day of the attacks.
They also released a second photo of his driving licence.
"The question I am asking the public, is did you see him at King's Cross?" said Peter Clarke, the head of Scotland Yard's anti-terrorism branch.
"Was he alone or with others. Do you know the route he took? Did you see him getting on the number 30 bus, and if so, where and when?"
Detectives are urging anyone who may have seen him during that hour to contact them on 0800 789 321 .
The four bombers had travelled together on the 7.40am train from Luton to King's Cross before going their different ways.
The 18-year-old from Holbeck, Leeds, killed 12 people on the number 30 bus in Tavistock Square.
The overall death toll has risen to 53 - from 52.
Detectives are also thought to be hunting a fifth British-born suspect - believed to have masterminded the plot - and an Egyptian chemistry student who may also have provided help.
The fifth man police are hunting is thought to be the bomb-maker or the orchestrator of the attacks, it is believed he is also a British Muslim with Pakistani heritage.
He is reported to have previous involvement in terrorist operations and may have links with al-Qa'eda followers in America.
The man is understood to be in his 30s, British and of Pakistani origin.
Detectives are said to believe he came into the country at a UK port last month.
It is not yet know if he left before the bombings took place or remained in Britain to offer final instructions.
Explosives have been found in an apparent "bomb factory" in Burley, Leeds and it is also feared the mastermind could have recruited a second bomb team.
A sixth man, thought to be called Magdi El-Nashar and a 33-year-old PhD biology student, today emerged as another person linked to last Thursday's attacks.
The Egyptian-born chemistry PhD student is thought to have handed over his flat in the Leeds suburb of Burley to the gang - possibly to the fifth man.
He vanished days before the bombings and it is not known if he was aware of the plot.
It emerged today that two of the bombers were cautioned by police for minor offences last year and it is also understood that one of the gang was looked at, but not arrested, during a major anti-terrorist operation in 2004.
A seventh home in Beeston, Leeds, is being investigated.