Police released pictures of Hassib Hussain, an 18-year-old from West Yorkshire in northern England, who police believe was responsible for the attack on a bus in Tavistock Square that killed 13 people.
Peter Clarke, head of Scotland Yard's anti-terrorist branch, also said police had identified a second suspect, who died in the blasts, and had evidence that two further, unidentified, suspects also died in the explosions.
He appealed for any information about Hussain - shown in one of the pictures carrying a large rucksack - in an effort to piece together his movements between leaving King's Cross and boarding the Number 30 bus.
The four were seen on security cameras at London's King's Cross station, heading off in different directions shortly before the blasts.
Police are continuing their
investigations across England
"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 on a Number 30 bus, if you did where and when was that?" he asked.
Metropolitan Police Commissioner Sir Ian Blair, Britain's most senior police officer, confirmed the identity of one of the suspects - a young Briton of Pakistani ancestry, already fingered by the press for his alleged role in the bombings.
"I can confirm the identity of the man who travelled from West Yorkshire and who died in the explosion in Aldgate," he said. "He was Shehzad Tanweer, aged 22. We believe he was responsible for carrying out that attack."
Clarke said a third, unidentified suspect had travelled from West Yorkshire to London, and that property in his name was found at the scene of the Aldgate and Edgware Road attacks - but no forensic evidence suggesting he was killed.
Based on forensic evidence, he said police believed they knew the identity of a fourth suspect who died in the explosion near King's Cross.
"We have just received forensic evidence that it is very likely that he died there," said Clarke.
Press reports have identified one of the other suspects as Mohammed Sadique Khan, 30.
Sky News television said the fourth was a Jamaican-born Briton named Lindsey Germail, 33, from Aylesbury, northwest of London.
"I can confirm the identity of the man who travelled from West Yorkshire and who died in the explosion in Aldgate"
Sir Ian Blair,
metropolitan police commissioner, London
A similar report appeared earlier in the day in The New York Times, citing "several American law enforcement officials" but using the spelling Lindsey Germaine.
A house in Aylesbury was the scene on Thursday of ongoing forensic investigations, as were premises in Leeds, in the north of England, that were raided by anti-terrorist police on Wednesday.
Aylesbury lies around 30km from Luton where the four bombers are believed to have boarded a train to the capital last Thursday.
It is thought the house in Northern Road in Aylesbury, Buckinghamshire, may be connected to one of the cars recovered by police at Luton railway station.
The bombs on the Underground trains - near Aldgate, Edgware Road and King's Cross stations - went off at around 8:50am (0750 GMT) last Thursday, with the one on the bus exploding nearly an hour later.
Aldgate, Edgware Road and King's Cross lie to the east, west and north of the heart of London.