Police on Wednesday said at least three of the bombers involved in bombing the London transport system are believed to be British males of Pakistani origin who lived in West Yorkshire in Leeds.
The bus bomber is believed to be dead, and police said there was "strong forensic and other evidence" a second bomber died at Aldgate. Investigators are now trying to establish if the other two are alive or died in the explosions.
A relative of one of the suspects was arrested in West Yorkshire on Tuesday and was being brought to London to be quizzed by the anti-terrorist branch.
Six search warrants were served on Tuesday under the Terrorism Act on houses in and around Leeds.
"These included the home addresses of three of the four men," an officer said. "A detailed forensic examination will now follow and this is likely to take time to complete."
The investigation has already established that personal documents bearing the names of three of the four men were found close to three of the explosions.
Armed officers and army bomb disposal experts took part in the pre-dawn raids on the properties in the Leeds area. Materials seized during the operation have been taken away for examination.
At least one controlled explosion was carried out ahead of a raid on one of the properties.
Around 500 to 600 people were evacuated from the area close to the address in the Burley district of Leeds at around 11.30 am.
A neighbour said a 22-year-old man had lived at the house on Hyde Park Road with his family but had gone missing.
A house in Lees Holm, Dewsbury, West Yorkshire, was one of those surrounded by police.
Neighbours said an Asian couple lived there with a young baby daughter. It is believed that the baby was aged about eight or nine months.
A neighbour said the couple had not been there for more than a year. She said the man was aged 29, while his wife was several years younger. The couple originally came from Pakistan but had moved from Leeds.
Earlier, police cordoned off a white semi-detached house in Colwyn Road, a quiet residential street in the Beeston area of the city, and a terrace house in Strateford Street, around two minutes' walk from Colwyn Road. Material seized during the raids have been taken away for examination.
The CCTV images showed the four men suspected had boarded a Thames link train at Luton, carrying rucksacks.
Sir Iqbal Sacranie of the Muslim Council of Britain said they have received the latest news from the police with "anguish, shock and horror."
He said, "It appears our youth have been involved in last week's horrific bombings against innocent people.
"While the police investigation continues we reiterate our absolute commitment and resolve to helping the police bring to justice all involved in this crime of mass murder. Nothing in Islam can ever justify the evil actions of the bombers."