Police are hunting the London bombers' accomplices, as it emerged explosives found in Leeds are similar to those used in other al-Qaeda-linked attacks.
Computers were seized from an office near a bomber's house
Explosives found in a house are thought to have been made from ingredients available from high-street chemists.
Police are now searching a Leeds house tied to an Egyptian chemistry student.
The BBC has also learned a suspected al-Qaeda member entered Britain two weeks before the bombs, but was not kept under watch.
A man injured in the bus bombing in Tavistock Square died in hospital on Thursday, taking the number of confirmed victims to 51. Three of the bombers have also been confirmed dead, with the fourth also thought to have died.
Police say the flat in Leeds being searched is linked to a 33-year-old Egyptian chemistry student, Magdi Mahmoud Elnashar, who colleagues have not seen since early July. He has not been formally confirmed to be a suspect.
The bombers' homes are also being searched.
Sources have told the BBC that the explosive recovered at a house in Leeds is home-made and that some of it remains in the house.
"This is a shocking development in the sense that earlier ideas about commercial or military grade explosive being used in the bombs themselves would therefore seem to be wrong," BBC Newsnight reporter Mark Urban said.
He said the explosive's "extremely volatile" nature had prompted the police to widen a cordon around the house even further on Thursday, as well as set up a no-fly zone.
Police said Hasib Hussain was shown carrying a rucksack
"It's the same kind of explosive Richard Reid had in his shoes when he tried to blow up a trans-Atlantic flight in 2001," he said.
Reid's version of the explosive had been mixed with another compound to make it more stable.
Anti-terrorist agencies are worried other "educated amateurs" could try to make more of the explosive, and that the risk of accidents in the manufacturing process is very high.
Police were also concerned that a man linked to al-Qaeda by intelligence reports had entered Britain via a Channel port before the bombings.
The man was on a security watchlist, but was not high enough on it to merit surveillance, Mark Urban said.
Authorities say the resources needed to keep a suspect under 24-hour surveillance are enormous and can only be used on selected targets.
Mohammad Sidique Khan (above): Aged 30, from Beeston, Leeds, recently moved to Dewsbury, married with baby. ID found at Edgware Road blast site.
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.
Shehzad Tanweer: Aged 22, born Bradford, lived Beeston, Leeds. Studied religion in Pakistan. Forensic evidence linking him to Aldgate blast.
Germaine Lindsay: Jamaican-born man who lived in Buckinghamshire.
"But inevitably people will now ask whether that suspect had any connection with the London bombings, why he wasn't further up the watchlist and whether port security is tight enough," our correspondent said.
He added that the man apparently left Britain hours before the blasts.
Speaking on BBC Radio 4's Today programme, Met Police commissioner Sir Ian Blair said there was nothing connecting the man to the plot, but added that investigators expected to find evidence linking al-Qaeda to the attacks.
He said the bombers were only the "foot soldiers" of the operation.
"What we've got to find is, who encouraged them, who trained them, and who's the chemist," he said.
He said police would also work with the Muslim community, which he said had been "close to denial" about extremist preachers, to defeat terrorism.
"We will break this horror that has descended upon us," he said.
Muslim community leaders are visiting mosques, charities and schools in West Yorkshire on Friday, including Sir Iqbal Sacranie, secretary-general of the Muslim Council of Britain (MCB).
Community sources report that the MCB delegation has met with relatives of one of the suicide bombers, Mohammad Sidique Khan, in Dewsbury.
The names of two bombers - Hasib Mir Hussain, 18, from Holbeck, Leeds and Shehzad Tanweer, 22, from Beeston, Leeds - were confirmed by police. Hussain blew up the Number 30 bus, while Tanweer struck the Aldgate Tube train.
Khan, 30, from Dewsbury, West Yorkshire, is believed to have blown up the Edgware Road train, though no forensic evidence as yet places him at the scene.
Police said DNA evidence from the forensic search of a house in Aylesbury, Bucks, would allow them to finally confirm the identity of the fourth bomber.
He has not been officially named, but police sources indicated he was almost certainly Jamaican-born Germaine Lindsay, who lived in Buckinghamshire.
Aylesbury is 20 miles from Luton, where the four bombers boarded a train which took them to London.
Susan Levy was a mother of two grown-up sons
On Thursday, police revealed a CCTV image of Hussain, captured at Luton rail station at 0720 BST, two-and-a-half hours before he blew up the bus.
Police are anxious to find out whether Hussain received last-minute orders to change his plans.
On Thursday, millions of people across Europe joined in a two-minute silence for the bomb victims.
A vigil attended by thousands of people was held later in Trafalgar Square in London.