The men who failed to blow up three London Tube trains and a bus last week are still in Britain and may have access to explosives, police believe.
Police have linked these men to the failed London bomb attacks
Three men being held by police are not thought to be the would-be bombers.
And a backpack carrying a device, which was found in a west London park at the weekend, has prompted speculation a fifth bomber may have been involved.
Armed officers have raided a north London property in connection with the events of 21 July but made no arrests.
They are still said to be searching the building in Ladderwood Road in the N11 postcode area.
It comes after Metropolitan Police Commissioner Sir Ian Blair said the investigation into the attempted bombings was "racing against time".
Tony Blair has appealed for anyone with information about who may have carried out last Thursday's attacks to come forward, saying it was everyone's duty to help protect the country.
BBC security correspondent Frank Gardner said the identity of those who carried out last Thursday's attacks was probably known by police.
He said there was a high probability the bombers still had access to explosives and that they wanted to use them.
Police were following a number of leads, including the possibility the groups that carried out the 21 July attacks and 7 July bombings, which killed 56 people, were linked, he said.
Police believe some of the 21 July team may have attended a pre-mission "bonding session" with 7 July bombers Mohammad Sidique Khan and Shehzad Tanweer, at the National Whitewater Centre, in Bala, north Wales.
Attention is focusing on whether the two groups were directed by the same people.
The international investigation is concentrating on Pakistan, rather than East Africa, as reported at the weekend.
There is nothing to suggest those behind last Thursday's attacks have fled the country, but police believe some of them may have killed themselves.
The backpack, found near Shepherd's Bush two days after the attacks, was destroyed after a series of controlled explosions.
It has prompted speculation that a fifth bomber may have been planning to take part last Thursday.
Scotland Yard would not confirm they were looking for a fifth bomber, but said they had always been looking for more than just the four suspects caught on CCTV.
Meanwhile, it has emerged that Brazilian Jean Charles de Menezes, 27, who was killed in error by police at Stockwell Tube station on Friday after they suspected he was a suicide bomber, had been in Britain on an out-of-date student visa.
'Victim of attacks'
The family of Mr Menezes say police apologies over his death are "not enough" and are considering suing them.
Sir Ian insisted that the policy of "shoot-to-kill to protect" would continue and warned that more innocent victims may be killed in the fight against terrorism.
London Mayor Ken Livingstone said Mr Menezes was a "victim of the terrorist attacks".
He added: "Consider the choice that faced police officers at Stockwell last Friday - and be glad you did not have to take it."
JEAN CHARLES DE MENEZES
Born 07/01/78, a Brazilian national
Originally from the town of Gonzaga, 500 miles northeast of Sao Paulo in the south-eastern state of Minas Gerais in Brazil
Moved to Sao Paulo at age 14
Lived in London for three years, working as an electrician
But Lord Ahmed said police must operate within "parameters of reasonableness" and warned that many Muslims were concerned they could fall victim to the "shoot-to-kill" policy.
Around 3,000 armed police are reported to be looking for the bombers on the streets of London and on the Underground network.
Police were given an extra 14 days on Sunday to quiz two men arrested in Stockwell on Friday on suspicion of preparing, instigating or commissioning terrorist acts.
On Monday, police will either release a third man, arrested in Tulse Hill, south London, on Saturday, or apply for another 14 days to question him.
The prime minister will discuss further anti-terror measures with the leaders of the main opposition parties at Downing Street on Tuesday.
A request from police to be able to detain terror suspects for three months without charge is expected to be on the agenda.
On Monday, Tony Blair visited doctors and nurses at London's University College Hospital who treated victims of the 7 July bombings.