The uncle of one of the suspected London suicide bombers said his family had been "left shattered" by the news.
Bashir Ahmed, 65, could not believe Shehzad Tanweer, who studied religion in Pakistan, was responsible. "It must have been forces behind him," he said.
Police hunting the masterminds behind the attacks are seeking a fifth person - not thought to be one of the bombers.
Anti-terror officers have also raided "residential premises" in Aylesbury, Bucks, in connection with the bombings.
Urgent action
No arrests have been made and no explosives recovered but forensic officers were examining the scene, BBC home affairs correspondent Margaret Gilmore said.
Aylesbury is 20 miles away from Luton, where the suspected bombers boarded a train into London on Thursday.
Meanwhile, Prime Minister Tony Blair has pledged urgent action to exclude and deport radical Muslim clerics who come to Britain to incite religious hatred.
Mr Blair said talks were to begin on new laws covering preparations for attacks and deporting people trying to "incite hatred".
He told MPs there would be "profound shock and anxiety" at the news that the suspected suicide bombers - who killed at least 52 and injured 700 - were British.
In other developments:
Detectives believe at least three British men of Pakistani descent died carrying out the attacks, the first of their kind in the UK. Police are yet to name the fourth suspected bomber.
At least three of the suspects are from West Yorkshire.
Relatives of one man - Hasib Hussain - had reported him missing last Thursday evening. Police sources told the BBC the 18-year-old was on the Number 30 bus.
One of the police raids on Tuesday - in the Beeston area - was on the home of 22-year-old Shehzad Tanweer.
Mr Ahmed said Shehzad Tanweer - who he last saw the day before the bombings - went to Pakistan for two months earlier this year to study religion.
Asked how he felt when police said his nephew had not been a victim but may have perpetrated the bombings Mr Ahmed broke down, saying: "We have lost everything we have."
A third man was named by newspapers as 30-year-old Mohammed Sidique Khan.
Shahid Malik, Labour MP for Dewsbury, where two homes were raided on Tuesday, told the House of Commons it was "the most profound challenge yet faced by the British Muslim community".
"Condemnation is not enough and British Muslims must, and I believe are prepared to, confront the voices of evil head on," he said.
Police believe two suspects died in the blasts at Aldgate/Liverpool Street and Edgware Road Tube stations, while a third died on the Number 30 bus at Tavistock Square. The fate of the fourth bomber is not yet known.
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.
The theory the bus bomber may initially have targeted the Northern Line, which on Thursday morning had been closed because of a defective train, is being pursued.
One house searched was used merely as an operational base for those involved in the bombings, and materials used in the attacks may have been stored there, according to local MP Greg Mulholland.
He said the house, in the Hyde Park area of Leeds, was not used as a family home.
Police are asking for anyone with information on the bombs to contact their anti-terrorist hotline on 0800 789 321
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