Hundreds of police and MI5 agents are continuing the hunt for the men who tried to bomb three London underground trains and a bus six days ago.
Officers are continuing to search a flat linked to the suspects
A flat in north London linked to two of the suspects is being searched, and police say chemicals found there and in a garage may have been for explosives.
Meanwhile, a neighbour has said she saw one of the suspects outside her home on the day after the bombings.
The Metropolitan Police chief said the men could carry out another attack.
Sir Ian Blair said the authorities were doing everything possible to find the men.
"They are capable of killing again," he told Channel Four News.
"We must find them. We are flat out and we are getting a great deal of intelligence."
Police have linked Yasin Hassan Omar, 24, and Muktar Said Ibrahim, 27 - also known as Muktar Mohammed Said - to the flat in the 12-storey Curtis House, in Ladderswood Way, New Southgate.
Police are hunting for Muktar Said Ibrahim (L) and Yasin Hassan Omar
A white VW Golf, seized in nearby East Finchley on Tuesday, may have been used by one of the suspects, security sources said.
Five people have so far been arrested in connection with the inquiry.
But none of the five, all held under anti-terror laws, is thought to have been among the failed bombers.
Detectives are investigating the possibility some of the suspects may have returned to the flat after Thursday's failed attack.
Neighbour Tanya Wright, who lives on the floor below Omar's flat, told BBC News she saw at least one of the suspects there in a group on Friday.
She said: "As soon as they saw me and the dog they looked very suspicious and very worried and just saw me and panicked and jumped back into the flat and slammed the door."
Detectives have no evidence the suspects have left the country and believe they may be hiding in a safe house in the capital. But there are fears they may have returned to the flat to retrieve bomb-making equipment.
WHO IS BEING HELD?
25 July - Man arrested near Curtis House, north London
24 July - Man arrested near Curtis House
24 July - Man arrested in Tulse Hill, south London
22 July - Two men arrested in Stockwell, south London
Police believe Omar tried to set off a bomb on a Victoria line Tube train between Warren Street and Oxford Circus on 21 July and Ibrahim was the bomber on a Number 26 bus in Hackney.
Both men have been legally resident in the UK for more than 10 years, having arrived as child dependents of Somali and Eritrean asylum seekers respectively, the Home Office said.
Two other men are being sought in connection with failed bombings - one near Shepherd's Bush, west London, the other near Oval station, south London.
There are also fears a fifth bomber is on the loose after a device was found in Little Wormwood Scrubs, west London, on Saturday.
TRACKS OF THE BOMB SUSPECTS
All journeys started between 12:20 and 12:25. Times approx.
OVAL: Man boards northbound Northern Line train at Stockwell and tries to set off bomb between Stockwell and Oval, where he leaves the train. He is chased out of the station at 1235 BST, but escapes towards Brixton.
HACKNEY: Man, identified by police as Muktar Said Ibrahim - or Muktar Mohammed Said - also sets off from Stockwell. Boards Number 26 bus at 1253 at Bank. Police believe he was carrying bomb in a grey and black rucksack, and tried to detonate bomb while on board. Gets off in Hackney Road, near junction with Colombia Road, at 1306.
WARREN STREET: Man who police believe is Yasin Hassan Omar boards Tube train at Stockwell carrying a purple rucksack that they say contained a bomb. Later tries to set off bomb on a northbound Victoria Line train between Oxford Circus and Warren Street, detectives say. Seen without rucksack at 1240 in Warren Street Station before running towards exit and vaulting over ticket barriers.
SHEPHERD'S BUSH: Man wearing dark blue baseball cap and carrying small rucksack enters Westbourne Park Tube station and gets a train travelling towards Shepherd's Bush on Hammersmith & City Line. Tries to set off bomb at 1225 before escaping, probably through window at the end of the carriage, and running along the tracks.