By Jeremy Scott-Joynt
BBC News business reporter
Changes in spending could help track the bombers' supporters
The identities of the four London bombers are now known.
But now comes the even harder part: trying to identify those who were responsible for sending them on their murderous mission.
According to Metropolitan Police anti-terrorist branch chief Peter Clarke, all the exhaustive work to date is just the start of the long task of identifying those responsible for sending the four to London.
"There are a number of things we need to establish," he told reporters. "Who supported them? Who financed them? Who trained them? Who encouraged them?"
Where to start?
Of these questions, the second could well prove to be the key to cracking the network open.
No-one can exist in the UK in the long term without leaving some kind of a financial trace behind.
Because of this, the fact that the bombers were British - however disturbing it may be - could at least make following the money a little easier, experts say.
One such is Dennis Lormel, who retired from the FBI in 2004 after almost three decades at the agency and is now a senior vice-president at Corporate Risk International in the US.
After years as a money laundering specialist he was the man who, on 12 September 2001, was charged with setting up the FBI's Terrorist Finance Operations Section to conduct the investigation into the finances of the 9/11 attackers.
"The first priority is the concern of whether there are going to be secondary attacks," he says.
That, he argues, is where financial investigations come into their own - particularly when you can start with known individuals.
"You build as comprehensive a financial profile as possible, and take it back as far as you can. Then connect it to communication records and so on, and you can put together a chronology.
"Between phones and finances, you'll see a lot of links to other people."
Among the raw data will be bank account details, credit card transactions - at least one of the bombers is believed to have been involved with credit card fraud, a common feature in recent bombings - corporate registry and charity records, as well as data from electoral rolls and police records.
And from that will emerge a spider's web of connections between the bombers on the one hand and people who have financed, supported or trained them on the other, generating a whole new set of leads for traditional investigations to take forward.
Some of what comes out of such an investigation will be innocent, Mr Lormel acknowledges. "But there should be enough intersects with those people who may be involved that something's going to stand out."
Focus on finance
On several occasions in the UK recently, this kind of probe has been the factor which has moved a suspect from being overlooked as a casual acquaintance to becoming a focus for the security services.
Financial evidence is just as important as physical evidence
Following the money is now a priority and is the responsibility of the UK's National Terrorist Finance Investigative Unit (NTFIU).
Set up after 9/11 within Special Branch, the arm of the police which works most closely with MI5 on security matters, the NTFIU is now the branch's fastest-growing unit.
It has been feverishly training financial investigators: those with the skills to pore over bank statements, corporate or charity accounts, ATM records and put them side by side with other information to draw up a "financial footprint" of their targets.
Increasingly, it has looked outside the police, bringing in people from the private sector to buttress the traditional investigative skills it already has.
All hands on deck
And elsewhere in law enforcement, it seems likely that the National Criminal Intelligence Service (NCIS), too, has thrown its staff into the hunt for the funding behind the bombers.
The NCIS has a small terrorist finance team, which develops intelligence for the NTFIU to exploit.
Far more numerous are its regular financial intelligence staff. They are responsible for the thousands of reports from banks, building societies, accountants, lawyers and even estate agents and casinos which are filed each week, warning of potentially suspicious transactions.
But on the day of the London bombings, the organisation's website carried a warning that NCIS Financial Intelligence was "redirecting many of its staff to other essential duties".
Some may have been put straight onto the investigation; others are believed to be digging through NCIS' huge backlog of suspicious activity reports (SARs) to check that nothing was missed.
"They'll have been told: we've got this huge stack of stuff," says Nick Kochan, author of several books on money laundering and terrorist finance.
"We can't be caught out if there's the slightest hint of a lead in there."
For Dennis Lormel, NCIS is simply doing what he would do.
"For at least the initial time, you are going to want to put every asset you have to contributing to the analytical product," he says.
Many of the records which need to be examined are on paper, or in incompatible formats. "It all needs to be put into databases - then you can start drawing out the connections."
This is the first feature in a series of three on the money trail which could lead to the London bombers' supporters. The others are to be published later this week.