Police have released images from CCTV footage showing one of the suicide bombers who took part in the attacks on London's public transport system last week.
Hussain (18) pictured in Luton station last Thursday
The image shows Hasib Mir Hussain (18) at Luton Station at 7.20am on the day of the bombings last Thursday.
He travelled to London city centre before blowing himself up on the No 30 bus in Tavistock Square along with 12 other people.
The head of Scotland Yard anti-terrorist branch, Peter Clarke, said police wanted to know what Hussain was doing in the hour after his three fellow bombers blew themselves up on tube trains. They detonated their devices at 8.50am but Hussain did not trigger his bomb until 9.47am.
"We know he travelled from West Yorkshire and arrived in London with three other men," Mr Clarke said.
"On the CCTV he was carrying a rucksack. We need to establish his movements up until 9.47am and up until the explosion occurred in Tavistock Square.
"What we are 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 on to a No 30 bus and if you did when and where was that.
"We are also keen to speak to everyone who was on that bus." He said he believed there were probably in the region of 80 people on the bus when the explosion occurred.
Mr Clarke publicly named two of the bombers for the first time. They were Hussain, from Holbeck, Leeds and Shehzad Tanweer (22), of Beeston, Leeds, who blew himself up on a train near Aldgate.
Another man, Mohammed Sadique Khan (30) from Dewsbury, West Yorkshire, and originally from Leeds, is believed to have blown himself up on a train near Edgware Road.
A driving licence picture of Hussain, also released by police today
Mr Clarke added: "In relation to the third and so far unidentified man, property in his name was found at the scene of the Aldgate and Edgware Road attacks but we are not yet able to get forensic evidence that he died in any of the explosions.
He said they now believed they know the identity of the fourth man who died at King's Cross station. "We have just received forensic evidence that it is very likely that he died there." This bomber was reported to be a Jamaican-born Briton in his thirties named Lindsey Germaine.
Police said today they may need months of investigations to uncover the planning and financial network behind the four young men who blew themselves up in Britain's first suicide attacks.
As police stepped up their search for witnesses, intensive efforts were under way behind the scenes to trace the missing members of the terror cell who carried out the bombing.
Detectives are hunting a fifth British-born suspect - believed to have masterminded the plot - who is thought to have arrived in Britain last month and left the country just days before the attacks. They are also searching for an Egyptian chemistry student who has recently gone missing from his home in Leeds.
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-Qaeda followers in America. The man is understood to be in his 30s, British and of Pakistani origin.