A number of other investigations are already under way, and civil lawsuits have been brought against the company involving alleged insider trading.

It is claimed the Enron managers were effectively hiding...$1bn of liabilities.

It seems likely the Justice Department will want to look into a number of secretive off-shoot companies set up to shoulder Enron's huge debts.

Enron's financial structure was certainly labyrinthine.

The question is - was it rotten to the core? The criminal investigation will seek to find out.

The company's collapse and bankruptcy - the biggest ever in the US - came about after confidence in the energy trader plunged, when news emerged about its huge debts.

The criminal inquiry may well cover the territory of some civil lawsuits brought against Enron, which allege that some of the top managers kept $1bn of liabilities from public knowledge.

It is also claimed that some of the management sold their own shares in the company when the share price was high, even though they knew about the extra liabilities waiting in the wings.

The lawyers bringing these cases say that amounts to insider trading, driven by false and misleading financial statements.

Hidden losses

The criminal case may centre on three so-called "partnerships" which did not appear in the company's accounts books.

These, it is alleged, were set up by some of Enron's top executives to offset risk - something that would normally be done using a third-party with no connection to the company.

It is claimed the Enron managers were effectively hiding the extra $1bn of liabilities, using these partnerships.

There are other investigations going on - four being carried out by Congress, one by the financial regulator, the Securities and Exchange Commission and one by the Labor Department, which is examining how the company's pension fund invested more than half its money in Enron's own shares.

The employees could not sell the holdings in their retirement accounts when the share price fell, and the pension fund has been badly damaged.