When Bernie Ebbers was ousted as WorldCom's chief executive in April in disgrace, it seemed only a matter of time before Scott Sullivan followed.

The telecommunications company's 40-year-old chief financial officer had formed a close partnership with the buccaneering Mr Ebbers as they turned the company into a leading force in the bull market. WorldCom had used a steadily rising stock price underpinned by rising earnings to take over several companies.

But John Sidgmore, WorldCom's new chief executive, spared Mr Sullivan. He seemed too important to the company to lose, exhibiting calmness under pressure, never losing his temper or being tripped up by a question from an analyst. He had an "extraordinary command of the facts and figures", says one person.

Just how extraordinary is now clear. This week, Mr Sullivan was abruptly fired by WorldCom's board, accused of the biggest accounting fraud in US history. He is said to have boosted the company's profits artificially by $3.8 billion (€3.75 billion) by counting some of its costs as capital investment. That helped the company to sustain its apparently smooth and rapid earnings growth.

The shock was all the greater because Mr Sullivan seemed to provide the trustworthy anchor to Mr Ebbers's flamboyance. He was the one that Wall Street and investors trusted - "the hardest worker in the history of the world" with "God's gift of being very bright", says one insider. It has left those around him struggling for an explanation.

Some suggest that Mr Sullivan may not have been able to resist the driving ambition of Mr Ebbers, who relied on WorldCom's share price being sustained so his stake would provide adequate security for a $400 million loan he had taken from the company.

"I think Bernie had a problem with ego and greed. Scott had a problem, not with Bernie, but with trying to please Bernie," says a former employee.

The WorldCom culture, say those who worked there, was all about living up to Mr Ebbers's demands. "You would have a budget, and he would mandate that you had to be 2 per cent under budget. Nothing else was acceptable - 'Go find a way to fix the problem'," says one. Another says: "Scott didn't want to admit to his mentor that he couldn't find a way to work the company through crisis, when the numbers started going south."

Mr Sullivan began his career as an auditor at KPMG, the same firm that nearly 20 years later would be given the task of trying to sort out the fraud he allegedly engineered. After graduating with high marks from Oswego State university in New York in 1983, he left the accounting world four years later for telecoms.

Mr Sullivan joined forces with Mr Ebbers after the WorldCom founder bought Florida-based Advanced Telecommunications, of which Mr Sullivan was treasurer.

They made an odd pair physically. Although he had wrestled at school, Mr Sullivan was dwarfed by Mr Ebbers's basketball-playing physique. While Mr Ebbers was happiest wearing his cowboy boots, Mr Sullivan stuck to suits.

He was named chief financial officer in 1994, a title replacing the more sober one of finance director. The CFO was supposed to be as much financial engineer as accountant. For Mr Sullivan, that meant ensuring WorldCom's acquisitions produced higher earnings and a rising share price, enabling it to buy yet more companies. To achieve this, he became a pioneer of "pro forma" financial reporting. This involved stripping out all the one-off (usually negative) items associated with a takeover to present what the company claimed was a truer picture of its underlying operating performance. Since the end of the bull market, such reshaping of figures has come under increasing scrutiny and criticism.

The deal-making also brought Mr Sullivan substantial personal rewards. Along with the $29 million in stock options that he cashed in during his years at WorldCom, he was paid substantial bonuses, including one of $10 million in 2000. Before the company's stock collapsed, his personal stake in it had reached about $200 million.

But there were drawbacks to working at WorldCom. Mr Sullivan hated Jackson, the Mississippi town where WorldCom was based. "He treated it like a Third World country and made fun of it every chance he got - though he hid it from Bernie," says one insider. Mr Sullivan never moved to Jackson, choosing instead to commute by air from his home in Boca Raton, Florida.

Mr Ebbers had his own gripes. According to someone who worked closely with both men, Mr Ebbers become jealous of the attention that Mr Sullivan was attracting. The financial whiz-kid had appeared on magazine covers and was praised for his brilliance. He was gaining a reputation for being the brains behind the operation but "Bernie only wanted himself to be the star".

Mr Sullivan worked a long day in an office next to that of Mr Ebbers, arriving at 8.30 a.m. and leaving at 9 p.m. He was generally respected, and used to sponsor WorldCom's participation in an annual Jackson walk for diabetes research. But his intensity could be off-putting, and he had little time for social pleasantries, prompting some to try to keep conversation "light and airy".

Nor was Mr Sullivan always a congenial colleague. Mr Ebbers frowned on senior managers openly criticising each other, but Mr Sullivan would sometimes talk critically of colleagues when the chief executive was not around. He took particular pride in having achieved so much so young. "He even tried to fake you out sometimes, pretend he was younger," says one insider.

The pressures on Mr Sullivan became more intense after WorldCom's $125 billion attempt to take over its rival Sprint failed in July, 2000. It marked the end of an era, and the telecoms sector went into a rapid decline soon afterwards. WorldCom alleges that the fraud started in early 2001. For now, it is impossible to tell exactly what happened at WorldCom - how the drive for growth turned into manipulating the numbers. But one insider believes it is a familiar tale. "Sullivan sold his soul to the devil," he says. "It's the old story - money and power."