They think he was a friend of the other three suicide attackers and, like them, lived an outwardly ordinary life in the Leeds area.
The men, at least three of whom were Britons of Pakistani origin, died with around 50 innocent victims when they blew themselves up on three packed rush hour underground trains and a bus.
Police and the security services fear the bombers could have been acting on the orders of an al-Qaida mastermind and there may be another bomb team waiting to strike.
Asked whether he believed they were part of a larger cell Home Secretary Charles Clarke said: "A central hypothesis which has to be tested and investigated is that the individuals we know about were working within a wider community."
Detectives were working furiously today piecing together the lives of the bombers as neighbours in West Yorkshire told of their shock that suicide attackers had been living in their midst.
The bombers had seemed liked normal young men who had lived in Britain all their lives.
Shehzad Tanweer, 22, from Beeston, Leeds, who blew himself up on the Aldgate train, was the son of a local fish and chip shop owner, he loved cricket and football and was a sports science graduate.
But friends claimed he had travelled to Afghanistan and Pakistan within the last six months, prompting fears he may have attended an al Qaida training camp.
Mohammed Sadique Khan, a 30-year-old from Dewsbury, West Yorkshire, blew himself up on the Edgware Road train, police believe.
Like Tanweer he seemed an unlikely suicide bomber. Friends said he was married with an eight-month-old baby girl and that he worked with disabled children in a primary school.
At almost exactly the same time - 8.50am - the as yet unnamed bomber blew himself up on a train between King's Cross and Russell Square.
Nearly an hour later Hasib Hussain, 19, who lived with his parents in the Leeds suburb of Holbeck, killed himself in the explosion on the number 30 bus at Tavistock Square.
When he left his home on Thursday morning, with only a few hours to live, he had told his parents the was going to London for the day with friends.
At 10.20pm that day his parents reported him missing to the police casualty bureau, providing one of the vital clues which led detectives to Leeds.
Neighbours said he had become "very religious" two years ago. His driving licence and cash cards were found in the mangled wreckage of the bus.