But how do you hide money on this scale? Is this a one-off or normal practice?

All these different alleged scams are confusing me. What is meant to have happened this time?

Investigators from a US Senate committee have found documents - emails and so on - that suggest Citigroup and JP Morgan Chase lent money to Enron while disguising the loans as something else.

The banks' story is that between 1992 and 2001 they paid $8.5bn to Enron in advance payment for energy trades furnished by Enron.

The money was channelled through a network of offshore companies the banks controlled.

"Dollars 5bn in prepays!!!!!!!" "Shut up and delete this email"

Email exchange between JP Morgan bankers

But the investigators say these so-called prepay operations were really no more than loans. Calling them trades meant that, to investors, they looked like sales.

And the banks raked in fat commissions from Enron, in the hundreds of millions of dollars.

Without the alleged subterfuge, according to the committee, Enron's debt in 2000 would have been $14bn, not the $10bn the company reported, and its cash flow would be just half the $3.2bn it claimed.

Pretty messy. But it's just those two, right?

Wrong.

Although Citigroup and JP Morgan Chase - two of the biggest names on Wall Street, incidentally - are those in the investigators' sights, they are not alone.

Also on the list is the UK's Royal Bank of Scotland and Barclays, as well as Credit Suisse and FleetBoston Financial, reported as having run up deals with Enron totalling about $1bn.

What is perhaps more interesting - if less than entirely surprising - is that both Citigroup and JP Morgan shopped the strategy to other firms. At least 10 similar deals were reached, the investigators say.

Energy firms in the US are falling over themselves to justify their own "prepay" trades, having seen their shares crash in Tuesday's trade as testimony to the committee went on.

Sounds like an open-and-shut case to me.

Opinions are divided on that.

The banks say the trades were legitimate, and were clearly marked as liabilities on Enron's books - even if they weren't actually written down as loans.

JP Morgan said the transactions were fully accounted for on its books under generally accepted accounting principles (GAAP).

The banks also say it's not their job to check Enron's own adding up.

Enron was assisted by Citicorp and Chase, said Senator Levin

Less convincing perhaps is their argument that their rectitude was confirmed in audits by Arthur Andersen, the discredited Enron accountant found guilty of obstructing justice.

Don't the banks have some difficult questions to answer?

Yes, some expert observers view the banks' justifications with a jaundiced eye.

For Steven Philippsohn, a fraud specialist from London-based lawyers Philippsohn Crawfords Berwald, the key question is how the "prepay" deals worked in practice.

"If these trades were real, and the money passed through, what did the trader get in return?" he told BBC News Online.

"If there wasn't anything concrete in return, then the banks are in serious trouble."

And the banks have yet to come up with a plausible explanation for why they were running deals like these, says Mr Philippsohn.

"These trades, so called, were conducted through offshore entities controlled by the banks.

"But what does a bank need with an offshore company like that? And what is it doing paying in advance for energy trades in any case?"

But where's the actual proof of bad practice? Surely you need some of that?

The investigators have spent seven months wading through more than a million pages of documents - and yet again US corporate types are discovering that email is both highly permanent and extremely embarrassing.

Thousands of pages of evidence were on display

The committee's investigators found messages which seem to confirm the bank's staff knew exactly what Enron was doing with the money.

One exchange between Citibank staffers, for instance, stresses that an oral agreement needs to stay that way.

"The paperwork cannot reflect their agreement," an email says, "as it would unfavourably alter the accounting."

Another one, involving bankers at JP Morgan, is even more succinct - and much more damning in an environment where destroying evidence is becoming the most heinous crime of all.

"Dollars 5bn in prepays!!!!!!!" one message reads, from an official who'd been adding up the exposure to Enron on his fingers.

"Shut up and delete this email," came the terse reply.

Worse still, an internal study Citigroup conducted in April 2001 found Enron was understating its liabilities by about $2.2bn - a fact the bank omitted to tell anyone else.

So what happens next?

That's where it gets sticky.

There are no signs yet of a Justice Department investigation into the behaviour of the banks.

And the general record of criminal charges against senior businessmen is pretty dismal.

But the civil courts are already filling up with class-action suits and other nasties directed at the banks for misleading investors.

About a dozen banks have been pursued to date, and one case - directed at Citigroup and Credit Suisse First Boston - links them to $2.5bn in debt sold at a time when, so the litigants protest, the banks must have known Enron was heading for disaster.

"If the banks knew there was debt - and there clearly was - and didn't mention it... it seems to me that they are liable for any loss sustained," Mr Philippsohn says.