A bath filled with explosives has been found at a house in Leeds that was the "operational base" for the London suicide bombers.
The discovery of a such a large amount of high explosives has shocked detectives and has raised fears of further attacks.
The news came as anti-terrorist officers were investigating a supposed fifth member of the terrorist gang thought to be on the run in Britain. It is unclear what role the suspect played, although he is not thought to be among the al-Qa'ida planners behind the attacks who are still at large.
Detectives disclosed on Tuesday that four suicide bombers all British-born men unknown to the security forces carried out the Tube and bus bombs that killed at least 52 people. Three of the terrorists have been identified as Shahzad Tanweer, 22, Mohammed Sadique Khan, 30, and Hasib Hussain, 18.
Police are believed to have obtained the name of the fourth Leeds-based bomber, who detonated a 10lb explosive in his rucksack on the Piccadilly line last Thursday morning, seconds after the train pulled out of King's Cross station, but have yet to formally identify him.
Anti-terrorist officers are focused on finding the rest of the terror unit, which is believed to include a bomb-maker. A security source warned: " We have not got the plotters and planners." Checks are being made at all ports, and MI5 is tracing all contacts made with the suicide bombers.
None of the bombers was on MI5's "watch list" people under investigation or suspected of al-Qa'ida involvement.
A security source, however, has revealed one of the bombers did come to the police's attention during an investigation into a plot to build a large bomb near London. The source said: "One [of the men] is linked in a very detached way, but he has no profile." The connection was described as "an association" which was considered low level.
Two of the three named bombers have single convictions for extremely minor crimes for which they received a caution.
Detectives, as part of the investigation codenamed Operation Thesis, traced the gang to the Leeds and Dewsbury area in West Yorkshire, from where the bombers travelled to London on Thursday morning. Raids on homes in the Leeds area on Tuesday morning discovered what a local MP yesterday described as an operational base for terrorists. It is understood that anti-terrorist officers found a bath filled with explosives at a house in the Hyde Park Road area of Burley, Leeds.
The explosive is being examined by specialists to discover ifit matches the high grade explosive used in the London bombs. It is not known yet whether the explosive used in the attacks was from a military or commercial source.
The huge quantity of explosive found, along with other explosives left in a hire car used by the bombers and abandoned at Luton train station, has led to fears that they may have made further bombs. Greg Mulholland, MP for Leeds North West, commenting about the house where the explosives were discovered, said: "It seems that this is an operating base for them. This is where the material may have been stored."
Army experts carried out a controlled explosion to get into the house, and 600 people were evacuated from the area as armed police raided the premises.
Meanwhile, anti-terrorist officers are continuing to piece together the movements of the bombers in the hours before they carried out the attack. They are examining CCTV footage taken from the railway stations and motorways which the terrorists are believed to have used. So far, they have established that at least three of the four travelled down from Leeds in one or two hire cars to Luton.
It is not known yet where the fourth man came from. He is thought most likely to have been a friend of the other bombers and to have been based in Leeds, but another possibility being suggested is that he was based in Luton.
The four men caught a Thameslink train to King's Cross station, where they were caught on film at about 8.30am on Thursday.
Three bombs were detonated on the Tube 20 minutes later; the fourth was set off after about 80 minutes on the No 30 bus in Tavistock Square.
The police hope that by finding CCTV of the bombers, they can trace them back to homes of other terrorists or capture them on film with fellow fanatics.
The security services are examining the background of the bombers, their friends and associates and looking at where they have travelled, both at home and abroad.
Detectives interviewing a 29-year-old man arrested in Leeds on Tuesday in connection with the London bombings have been granted a further three days to question him. The suspect is related to one of the bombers.
Anti-terror police carried out a raid on a house last night in Aylesbury, Buckinghamshire. Scotland Yard said this was in connection with the bombings. Niknam Hussain, a local councillor, said that he understood from police that the property was occupied by a man who had "connections with a car in Luton".
According to residents, the raid centred on the junction between Northern Road and Bicester Road. No arrests were made.
Detectives interviewing a 29-year-old man arrested yesterday in connection with the London bombings have been granted a further three days to question him. The suspect is related to one of the bombers.
Assistant Commissioner Andy Hayman
A former chief constable of Norfolk, Andy Hayman, 45, has made an impressive start since being named the new head of Scotland Yard's Specialist Operations section in January. Mr Hayman is responsible for the counter-terrorism department, Special Branch and the royalty and diplomatic protection teams. He joinedEssex Police in 1978. His role is a crucial interface between the police and the Security Services, because it is the police Special Branch which carries out operational counter- terrorism work.
Deputy Assistant Commissioner Peter Clarke
The policeman heading the hunt for the bombers is a 49-year-old detective who used to be in charge of the Met's Royalty and Diplomatic Protection Department. Deputy Assistant Commissioner Peter Clarke took over the role as the national co-ordinator of UK anti-terrorism operations, in addition to heading the Met's Anti-Terrorist Branch, in May 2002. He joined the Met in 1977 after gaining a law degree at Bristol University. He is highly rated by fellow officers and the Muslim community and is married with three children.
The questions still to be answered
* Why did Hasib Mir Hussain, 18, set off his bomb on a bus nearly an hour after his three fellow bombers detonated their devices on Tube trains? One theory is he was due to head north on the Northern line, but it was shut, so he caught a bus. But why wait nearly an hour?
* Why did the terrorists leave so much explosive back at their bomb factory in Leeds? Why not hide it or allow others to take it away?
* What was the explosives used in the bombs? The substances come from either military or commercial sources. We know the explosive used was of high grade, which suggests it was from a military source but the forensic analysis has not yet been completed. Once the source has been identified it can help officers trace where it came from.
* Where did the fourth bomber come from when he met up with the others at Luton train station? He is most likely to have been in Leeds and travelled down in a hire car, but until CCTV footage has been analysed we will not know. One alternative is he came from Luton, which begs the question - where was he staying?
* Who helped the terror unit? That is what the security services are trying to establish by looking at all the contacts - by telephone, via computer, and in person - the men have made. Analysis of the bomb-making factory is also important as it might have fingerprints, DNA, or other clues pointing to the identity of other extremists.
* Why has it been so difficult to identify the fourth bomber? The bomb he is thought to have detonated was on the Piccadilly line in a very narrow tunnel between King's Cross and Russell Square, which caused massive damage and blew the killer into small pieces.
* Who was responsible for radicalising the bombers? Investigations into the background of the men, both during their time in Britain and travelling abroad, notably to Pakistan, should give some indication.
* Are there other extremists with rucksack bombs from the same team ready to carry out attacks? That is the question all of Britain wants to know. But until detectives and MI5 have traced the movements of the known bombers, we have little chance of finding out.