At least 54 people were killed and hundreds of others injured in the suicide bombings on 7 July on three underground trains and a double-decker bus.
Three of the four suspected bombers were of ethnic Pakistani origin. The fourth bomber has been identified as a Jamaican-born Briton.
The head of the London Metropolitan Police antiterrorist branch, Peter Clarke, says investigators are focusing their probe on who may have helped plan and finance the attacks.
"We need to establish a number of things: who actually committed the attack, who supported them, who financed them, who trained them, who encouraged them. This will take many months of intensive detailed investigation," Clarke said.
Pakistan's Foreign Ministry said yesterday that it is awaiting details of when any of the suspected bombers may have visited Pakistan.
Detectives are continuing their searches and forensic examinations of the homes of the suspected bombers -- in the northern city of Leeds, and in Aylesbury, near London.