The US Justice Department has launched an investigation into fallen energy giant Enron, joining Congress and other government agencies probing the company's financial dealings, officials said.

The probe will be carried out by a task force that includes US attorneys from Houston, New York, San Francisco and other parts of the country, who will look into whether company executives committed fraud before Enron went bankrupt late last year.

The Justice Department's Criminal Division in Washington will have overall control over the investigation. The decision puts the Justice Department in the company of the Securities and Exchange Commission, the Labour Department and several US congressional committees looking into the dealings of the troubled corporate colossus, once the largest energy trading company in the world.

The Houston-based firm was President Bush's biggest corporate backer when he ran for governor of Texas. It has since advised the Bush administration on energy policy.

The firm's filing for Chapter 11 bankruptcy protection on December 2 marked the largest-ever bankruptcy filing of a US corporation.

Enron's troubles began after it revealed that it had been keeping financial losses - incurred in relationships with private partnerships run by some of its own corporate officers - off its balance sheet. The one-time star stock performer is carrying about $13 billion in balance sheet debt and owes billions more in loans, credit line repayments and payments on energy contracts.

The much publicised corporate collapse was then followed by a string of allegations of shady deals and questionable payments that prompted investigators to take a closer look at the Texas firm. It was reported, for example, that prior to filing for bankruptcy, Enron paid out $55 million in bonuses to about 500 employees.

Many Enron employees have also complained that they were encouraged to buy company stock and then prevented from selling when its price went south, while senior executives were allowed to get rid of their stock holdings when their value was still attractive.

* US President George W Bush has denied discussing Enron's financial woes with chief executive Ken Lay, who was a generous donor to Bush's campaign. Bush promised a full investigation into the Enron collapse.