By David Schepp BBC News Online North America business reporter

The arrest of Andrew Fastow, who formerly oversaw the books at Enron, has left many wondering which other big fish at the disgraced energy giant may next face arrest.

[I]t's pretty clear that a tremendous amount of the information upon which the indictment is based came from Kopper himself

J Boyd Page attorney

Some point to former chairman Kenneth Lay, who fashioned Enron into the corporate giant it became, or Jeffrey Skilling, who served as chief executive when Mr Fastow began his allegedly murky accounting.

But experts say the significance of Wednesday's arrest cannot be overstated.

"By the time you get to Fastow, there's only two other guys left," says Houston attorney Thomas Ajamie, an expert in complex securities matters.

Out of the tens of thousands of Enron employees, the government has managed to work its way up the food chain to Messrs Fastow, Lay and Skilling, Mr Ajamie says.

The falling domino

Mr Fastow served as Enron's chief financial officer and as such supervised Enron's finances.

"We know it was the finances and the financial dealings that caused the collapse of Enron," Mr Ajamie says.

Experts say Mr Kopper's arrest led to the charges against Mr Fastow

In surrendering to federal authorities Mr Fastow now faces multiple charges of fraud and conspiracy to commit fraud.

It was the August arrest of his close aide Michael Kopper, who pleaded guilty to conspiracy charges, experts say led to charges being filed against Mr Fastow.

As part of a plea deal, Mr Kopper agreed to cooperate with prosecutors in their investigation.

Mr Kopper worked in Enron's Global Finance unit and took a lead role in the formation of the off-balance-sheet partnerships that led to Enron's collapse.

"If you read the [criminal complaint], it's pretty clear that a tremendous amount of the information upon which the indictment is based came from Kopper himself," says Atlanta attorney J Boyd Page, whose firm represents defrauded investors.

'Telling language'

Mr Kopper's assistance was essential in building this specific and extensive an indictment, experts say.

Some wonder if Kenneth Lay will be next

Mr Fastow is accused of making huge personal gains - in excess of $30m - from his alleged misconduct.

For that reason legal experts do not believe that Mr Fastow represents just another step up the Enron hierarchy.

"He is and always has been one of the true focal points of the investigation," Mr Page says.

Nonetheless, the government's complaint does make references to other Enron officials, but only generically.

"That kind of language is very, very telling," says Mr Boyd. "It's just an interesting way of phrasing things."

He says it suggests Messrs Skilling and Lay are being scrutinised very closely.

"To me that suggests this is far from over."

Jail time?

Mr Fastow may be able to arrange for a more favourable plea if he implicates other senior executives or directors in connection with the ongoing investigation.

That does not seem likely, however, following a statement from Department of Justice lead prosecutor Andrew Weissmann.

He told reporters in Houston on Wednesday that Mr Fastow was not cooperating in the investigation.

Asked if he thinks Mr Fastow would serve time in prison, Mr Boyd says it is often difficult to prove before a jury guilt beyond a reasonable doubt in white-collar crimes.

Nevertheless, he says, "I have to believe there is a strong likelihood that Fastow will serve time."