Many questions remain about energy giant's collapse, writes Denis Staunton in New York

After 15 weeks of testimony from more than 50 witnesses, jurors will return to a Houston courtroom next Monday for closing arguments in the trial of former Enron bosses Kenneth Lay and Jeffrey Skilling.

The two defendants spent four weeks on the witness stand but, as the trial winds up, many questions about the energy giant's collapse remain unanswered.

Key figures in the company who could have corroborated or demolished the testimony of Mr Lay and Mr Skilling refused to appear because they feared incriminating themselves in advance of their own criminal trials.

Defence lawyers are likely to focus on these missing witnesses next week as they make a final effort to save their clients from convictions that could send them to prison for 25 years.

Mr Lay, who founded Enron, and his successor as chief executive, Jeffrey Skilling, are charged with multiple counts of conspiracy and fraud in connection with the 2001 collapse of the energy giant, which was once America's seventh-largest company. Prosecutors say the two men deceived investors and the public about the true state of Enron's finances and authorised illegal stratagems to massage earnings.

Before the trial began, many observers predicted that the prickly Mr Skilling, who has a reputation for arrogance, could prove a liability for both defendants by alienating jurors with his abrasive style.

Instead, Mr Skilling was a model of self-discipline in the witness box, answering most questions calmly and even admonishing himself when he showed a rare flash of irritation.

Mr Lay, by contrast, was on the defensive most of the time, belying his sunny-tempered image by snapping at lawyers and making sour jokes that fell flat with jurors.

Prosecution witnesses said Mr Lay and Mr Skilling had created a culture of corruption at Enron in which the company had to reach profit targets by any means necessary.

The witnesses said the defendants approved dubious transactions to inflate quarterly earnings and talked up Enron's performance at a time they knew the firm was in trouble.

The defence case is that Mr Lay and Mr Skilling are not only innocent of the charges they face but that there was no crime committed at Enron at all, apart from the millions of dollars former financial chief Andrew Fastow siphoned off from the company.

Defence lawyers contend that the judge's refusal to offer immunity from prosecution to some witnesses has hampered their case."A trial is supposed to be a search for truth. Had these people been here, the facts would have been more complete and that much clearer to this jury," Daniel Petrocelli, Mr Skilling's lead lawyer, said.

Without any high-level executives to support them in court, Mr Lay and Mr Skilling had to persuade the jury during their own testimony that Enron was brought down by greedy traders and a nervous market alarmed by revelations of Mr Fastow's misdeeds.

Mr Skilling portrayed Enron's activities as more complicated and nuanced than the prosecution claimed, lending some credibility to his claim that he relied on the advice of internal accountants and lawyers.

Mr Lay, by contrast, stumbled and snarled as he attempted to explain why he sold $70 million (€54 million) in Enron shares a few months before the company collapsed, at a time when he was telling employees the shares were a bargain.

Mr Lay said he did not feel the need to tell employees, many of whom lost their retirement savings after Enron's collapse, that he was selling shares.

Mr Lay's share sell-off is significant because he and Mr Skilling have blamed the company's collapse on "vultures", who became short-sellers of Enron shares when scandal started to break.

Despite some predictions, Mr Skilling's testimony did nothing to damage Mr Lay but Mr Lay's statements could serve to implicate his successor.

When asked to identify his biggest mistake as Enron boss, Mr Lay said it was hiring Andrew Fastow, who has admitted fraud and is co-operating with the prosecution.

In fact, it was Mr Skilling who hired Mr Fastow, a detail that will not have escaped the jury.

Praising Mr Skilling as an executive who "really gets into the details, the guts of how things work", Mr Lay signalled that his successor had a firm grip on activities within the company.

The Enron founder stressed at the start of his testimony that he was a delegator who left the details of the business to subordinates.

Most legal experts believe that the defence has failed to make a strong enough case to save Mr Lay and Mr Skilling from conviction, although the prosecution case is weakened by the fact that many of its key witnesses have made plea bargains in return for their testimony.

The trial judge Sim Lake handed prosecutors a strategic victory on Wednesday, saying he would tell jurors that "deliberate ignorance" of fraud at the collapsed energy company was not a justifiable defence.

Legal experts said it would increase the chance of guilty verdicts.

Judge Lake's ruling will allow the jury "to take a second bite at the apple" when deciding whether Mr Lay and Mr Skilling are guilty, according to Jonathan Halpern, a partner at law firm Winston & Strawn in New York and a former federal prosecutor.

If the jury agrees with the prosecution's argument that fraud occurred at Enron under Lay and Skilling, he said, they would not need to decide whether the two had direct knowledge of it, only that the two executives should have been aware of it.

When testimony ended last week, both Mr Lay and Mr Skilling sounded upbeat, saying they were pleased the trial was drawing to a close.

"We feel real good about where we are right now. We think, in fact, in the end we're going to prevail," Mr Lay said. - (Additional reporting, Reuters)