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.
No arrests have been made and no explosives recovered but forensic officers were examining the scene, BBC home affairs correspondent Margaret Gilmore said.
Shehzad Tanweer, 22, was born in Bradford to Pakistani-born parents
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:
- Police were granted a warrant allowing them to hold a 29-year-old relative of one of the suspects - arrested on suspicion of commissioning, instigating or preparing acts of terrorism - until Saturday
- Home Secretary Charles Clarke vehemently denied claims by French Interior Minister Nicolas Sarkozy that some of the suspects had been arrested in 2004 but were freed in a bid to catch a wider network
- Scotland Yard said the Identification Commission had confirmed the identities of 14 victims - 11 of whom have now been named
- All countries in the EU said they would join the UK in holding a two-minute silence on Thursday to remember the victims of the London attacks.
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.
Shehzad Tanweer: Aged 22, born Bradford, lived Beeston, Leeds. Studied religion in Pakistan. Forensic evidence linking him to Aldgate blast
Hasib Mir Hussain: Aged 18, lived Holbeck, Leeds. Reported missing on day of bombings. Said to have turned very religious two years ago. ID found in No 30 bus
Mohammed Sidique Khan: Aged 30, from Beeston, Leeds, recently moved to Dewsbury, married with baby. ID found at Edgware Road blast site.
Fourth bomber: Still a question mark over his identity. Believed to be a friend of the others and from same area.
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.
1 The four men arrive at King's Cross Thameslink station from Luton. They go to the Tube station and fan out on different lines
2 Shehzad Tanweer takes the Circle line eastbound. Bomb explodes between Liverpool St and Aldgate
3 An unnamed suspect takes the Piccadilly line south. The train blows up before the first stop at Russell Square
4 Mohammed Sidique Khan takes the Circle line westbound. Bomb explodes at Edgware Rd
5 Hasib Hussain may have tried the Northern line but he ends up on the No. 30 bus. Bomb explodes at Tavistock Square