Police have issued a CCTV image and a photo of the man they say carried out the Number 30 bus bombing in London last week, in which 13 others died.
Police said Hasib Hussain was shown carrying a rucksack
Scotland Yard urged anyone who had seen Hasib Hussain last Thursday to call the anti-terrorist hotline on 0800 789 321.
Hussain, 18, and from Leeds, was seen on CCTV in Luton, carrying a rucksack, before he boarded a train to London.
Detectives urged the public to help them trace his movements before the bomb went off in Tavistock Square.
Police said on Thursday that a man injured in that blast, which happened at 0947 BST, had now died in hospital, taking the death toll from all four bombings to 54.
Police say three of the four bombers thought to have died in the attacks are included in this figure.
A two-minute silence was earlier held in tribute to all those killed and injured in the blasts at 1200 BST.
An evening vigil was also been held in Trafalgar Square, central London.
Police have been searching properties in Aylesbury, Bucks, and Leeds.
A controlled explosion has been carried out and residents moved in the Beeston area of Leeds, where one of the bombers used to live.
Head of the anti-terrorist Branch Peter Clarke said: "We need to establish [Hussain's] movements up until 0947 BST, when the explosion occurred in Tavistock Square," he said.
"The question I am asking the public is: 'Did you see this man at King's Cross, was he alone or with others?
"Do you know the route he took from the station, did you see him get onto a number 30 bus, and if you did - when and where was that?"
Police think there were about 80 people on the bus when the explosion happened and Mr Clarke called for any passengers who had not contacted the police to do so.
He told a police conference in London the police investigation had uncovered a "vast amount of information", with new leads emerging "literally by the hour".
He said detectives had taken 500 witness statements and had seized - and were analysing - more than 5,000 CCTV tapes.
"We need to establish a number of things: Who actually committed the act, who supported them, who financed them, who trained them, who encouraged them?
"This will take many months of intensive, detailed investigation."
He said forensic investigations at the scenes of the attacks were continuing, while properties were being searched in West Yorkshire and in Aylesbury.
Mohammad Sidique Khan: Aged 30, from Beeston, Leeds, recently moved to Dewsbury, married with baby. ID found at Edgware Road blast site.
Hasib Mir Hussain: Aged 18, lived Holbeck, Leeds. Reported missing on day of bombings. Said to have turned very religious two years ago. ID found in No 30 bus.
Shehzad Tanweer: Aged 22, born Bradford, lived Beeston, Leeds. Studied religion in Pakistan. Forensic evidence linking him to Aldgate blast.
Germaine Lindsay: Jamaican-born man who lived in Buckinghamshire.
Security sources have told the BBC that the Aylesbury search is connected with trying to identify the bombers.
In particular, detectives are trying to establish if the fourth bomber had either lived or stayed there.
Police sources have indicated he was Jamaican-born Germaine Lindsay, who lived in Buckinghamshire.
Aylesbury is 20 miles from Luton, where the four bombers boarded a train which took them to London.
Police are also trying to find the source of explosives found on Tuesday in a raid on a property in Leeds and in a car parked at Luton Central railway station. A second car found at Luton is also being examined.
Mr Clarke also confirmed the identity of the second bomber, Shehzad Tanweer, 22, who died in the explosion at Aldgate.
He said property belonging to the third man who travelled from West Yorkshire was found at the scenes of the Aldgate and Edgware Road attacks.
But he said there was not yet forensic evidence that he died in the blasts.
The man has been named as Mohammad Sidique Khan, 30, from Beeston in Leeds.
However, Mr Clarke said forensic evidence did indicate that the fourth bomber, Germaine Lindsay, had died in the Russell Square blast.
Police are also hunting a fifth man who they believe masterminded the attacks. It is believed he left Britain shortly before the bombings.
A sixth man, an Egyptian chemistry student who has disappeared from his house in Leeds, is also being sought.
1 The four men arrive at King's Cross Thameslink station from Luton. They go to the Tube station and fan out on different lines
2 Shehzad Tanweer takes the Circle line eastbound. Bomb explodes between Liverpool St and Aldgate
3 An unnamed suspect takes the Piccadilly line south. The train blows up before the first stop at Russell Square
4 Mohammad Sidique Khan takes the Circle line westbound. Bomb explodes at Edgware Rd
5 Hasib Hussain may have tried the Northern line but he ends up on the No. 30 bus. Bomb explodes at Tavistock Square