Police investigating the London bombs are now focusing on finding those who masterminded the suspected suicide attacks that have killed at least 52.
Police are continuing to search premises in Leeds
Detectives believe three British men of Pakistani descent died carrying out the first attacks of their kind in the UK.
A fourth bomber, from Leeds, has been identified by police. No further details have been released.
One man was arrested in West Yorkshire, where three of the suspects, yet to be officially named, were from.
Terrorism experts say the men may have been guided by a "controlling hand".
Police believe two of the 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, and the fourth at King's Cross.
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.
Metropolitan Police Commissioner Sir Ian Blair has said another bomb attack is "likely" and there are fears that a second suicide bomb gang may be preparing to strike.
The attacks in the morning rush-hour last Thursday killed at least 52 people, and injured more than 700.
Scotland Yard on Wednesday named six more victims of the bombs, bringing the total of identified bodies to 11.
Police sources told the BBC they had not recovered any timing devices from the bomb scenes, possibly indicating that detonation was by hand.
They are also investigating the theory that the bus bomb may initially have been targeted at the Northern Line, which on Thursday morning had been closed because of a defective train.
A relative of one of the West Yorkshire suspects was arrested and taken to London for questioning.
It emerged that relatives of one of the men - Hasib Hussain - had reported him missing last Thursday.
Police sources told the BBC that the 18-year-old was on the Number 30 bus.
One of the police raids - in the Beeston area - was on the home of 22-year-old Shehzad Tanweer, who has not been seen for a few days.
One local resident described him as "a nice lad".
"He liked to play football, he liked to play cricket. I'm shocked."
Shehzad Tanweer, 22, was born in Bradford to Pakistani-born parents
Another resident said he was just a "normal kid" who played basketball and kicked a ball around.
A third man was named by newspapers as Mohammed Sadique Khan, 30.
On Monday night, police had viewed CCTV footage of the four suspects together at London King's Cross last Thursday.
All had rucksacks and were seen 20 minutes before the three Tube bombs started going off at 0851 BST. The bus bomb went off at 0947 BST.
Three of the men had travelled to Luton from Leeds by car, before catching a Thameslink train to London. They were joined at Luton by a fourth man, believed to have driven to the Bedfordshire town.
Tuesday's police raids, which began at 0630 BST, centred on two properties in Dewsbury, West Yorkshire, and four in Leeds.
Explosives were also found in a car at Luton railway station and a number of controlled explosions were carried out. The car has now been taken away for further examination.
London: Forensic work ongoing at blast sites
Leeds area: Six houses searched, controlled explosion at one, one arrest made
Luton: Controlled explosion after car find near station
A second car believed to be linked to the attacks was also found at the station and towed to Leighton Buzzard, 10 miles (16km) west of Luton, for further examination.
Sir Iqbal Sacranie, of the Muslim Council of Britain, said it had received news about the suspects with "anguish, shock and horror".
He said: "It appears our youth have been involved in last week's horrific bombings against innocent people.
"Nothing in Islam can ever justify the evil actions of the bombers."
Metropolitan Police Assistant Commissioner Andy Hayman said: "I want to make it absolutely clear that no-one should be in any doubt the work last Thursday is that of extremists and criminals.
"No-one should smear or stigmatise any community with these acts."
Metropolitan Police Commissioner Sir Ian Blair said the searches, carried out under the Anti-Terrorism Act, were intelligence-led and "directly connected" to last week's attacks.
Peter Clarke, head of the Met's anti-terrorist branch, said of the suspects: "We are trying to establish their movements in the run up to last week's attacks and specifically to establish if they all died in the explosions."
Police are asking for anyone with information on the bombs to contact their anti-terrorist hotline on 0800 789 321.
LONDON BOMBINGS EVIDENCE
King's Cross railway station
Four men, three of them from West Yorkshire caught on CCTV at 0830 on morning of attacks
Property of Suspect 1, from W Yorks, found in bus wreckage
Liverpool Street / Aldgate
"Very likely" that body of one suspect from W Yorks found
Property of Suspects 2 and 3 found
Property of Suspect 3 found