Anti-terror officers from Scotland Yard are investigating how one of the 7 July bombers, who worked part-time in a fish and chip shop, left more than £100,000 after his death.
Senior police sources say that they are working on the theory that Shehzad Tanweer, who blew himself up at Aldgate tube station, may have been paid by a terror mastermind to carry out the attacks.
The mystery of the 22-year-old bomber's finances has emerged six months on from the London tube and bus attacks which left 52 dead and wounded more than 700.
Detectives and intelligence agencies have so far failed to discover who, if anyone, was responsible for orchestrating the 7 July blasts, despite analysing hours of CCTV footage, phone records and forensic science evidence.
The Independent on Sunday has learned that financial experts had already investigated the financial backgrounds of Tanweer and his fellow bombers, Mohammad Sidique Khan, Hasib Hussain and Jermaine Lindsay, to establish if they were involved in money-laundering or with international criminal networks.
However, the priority had been placed on looking into what financial transactions they had carried out, not the value of their individual assets.
A police source said that the £121,000 left by Tanweer, which was documented in legal papers, could be the result of a gambling habit or he might have been left a property by a relative. But they the fact he had the money was still considered to be significant.
"We have got experts probing their financial affairs and looking at their financial profiles but this was a revelation for us," they said. "This will be looked into it to see if there is any link with crime."
Tanweer worked in his parents' takeaway business in the Leeds suburb of Beeston and had dropped out of a sports degree at Leeds Metropolitan University. In common with the three other London bombers, he is known to have visited Pakistan.
A team of police investigators has been sent to the country to work alongside the security services in an attempt to investigate whom the bombers met and who trained them to carry out the attacks. Although they have numerous possible suspects, a definitive list has yet to be drawn up because of a lack of intelligence.
The forensic evidence that was gathered in the wake of the 7 July bombing and the failed 21 July attacks has also failed to establish any link between the two sets of bombers. Police sources said it had now been clearly established that the explosives bought for the later attacks were bought before the 7 July atrocities. This means they are no further forward in identifying the mastermind who might have trained either sets of bombers.
The Metropolitan police have confirmed that they have thwarted at least three terror plots since July. This paper has also learned that in the weeks after the July attacks, the security services warned Tony Blair, the Prime Minister, of a new and specific terror threat on London. But he took the decision not to shut down the tube networks, as had been suggested, after a high-level meeting with police chiefs and MI5.
There is huge concern that Britain may again become the target of extremists desperate to regain support for their cause after the bombing of a wedding party at a hotel in Jordan which was widely condemned.
Forces around the country, especially the West Midlands and Greater Manchester, have also been warned to be on the alert.
"It is still a question of when [another attack happens] rather than if. Jordan was seen as a disaster for the terrorists and they will be keen to regain credibility, either by an attack on the UK or US," said a senior Met insider.
Extremists are increasingly making use of the internet in an effort to avoid detection by police. A new tactic is for terror "commanders" to set up encrypted websites, accessible only by a secret code, which give details of how to plan and orchestrate an attack. This means that the " foot soldiers" - fanatics recruited to the terrorist cause - do not have to communicate with the rest of their cell directly via mobile phones or other conventional means. Instead, they meet up only on the day of the planned attack.
As a result, police investigators are recruiting more computer experts who can help them to break into these sites and thwart terror attacks before they take place.
Later this month, the Met will learn whether investigators are to recommend that charges be brought against any of its officers involved in the shooting of Jean Charles de Menezes at Stockwell tube station. The Independent Police Complaints Commission (IPCC) is expected to send the Crown Prosecution Service its conclusions into the killing of the Brazilian electrician, shot by mistake when he was misidentified as one of the 21 July terrorists.
Although a report has already been drafted, it is understood that the IPCC has not decided yet whether or not to recommend that charges should be brought against any of the officers involved.
Still waiting for answers six months after 7 July
1: Were the attacks prompted by the Iraq war? Mohammad Sidique Khan claimed in a video message that the attacks were in response to " atrocities" committed by the West against Muslims. This was roundly denied by ministers although there has been no conclusive probe into the reasons behind the attacks.
2: Was there a mastermind behind the London bombings? The suicide bombers did go white-water rafting in Wales with a visitor from Pakistan. However, police have since dismissed the relevance of the trip. Anti-terror officers remain "open-minded" about the possibility of the existence of a terror "commander".
3: Was there a fifth bomber? Police did find an unused rucksack full of explosives in the men's car which had been left at Luton station. A fifth suspect was arrested in Egypt shortly after the attacks but later released without charge.
4: Why was the terror alert downgraded before July? British intelligence agents decided that al-Qai'da did not have the capacity to carry out an attack on the UK, so ministers were advised to lower the alert status. On the day of the 7/7 bombings, the focus was on providing security for the G8 summit.
5: Were the 7/7 and 21/7 attacks linked? Initially police said they would be surprised if there was no connection given the similarity in the numbers of bombers involved and their targets. However, no evidence has emerged to support this theory.
6: How did the 7/7 bombers remain undetected before their attack? Khan, regarded as the ringleader of the 7/7 cell, was known to the security services and it has been reported that they had also looked into the background of Tanweer. But neither was considered to be a serious or credible threat even though they had travelled to Pakistan.
7: Was it true that the suicide bombers had been arrested three years before the London attacks? Nicolas Sarkozy, the French interior minister, claimed that members of the 7/7 cell had been subject to a partial arrest in 2002. This was quickly denied by Charles Clarke, the Home Secretary, who said he was not "personally aware" of any such event.