There were high hopes for Susan Rivera when she was hired away from AIG, the world's largest commercial insurer, in 2002 to take charge of the US business of Bermuda-based ACE Limited.

But hopes sometimes fade - and it would appear they did so almost overnight in this case with the sudden announcement on Wednesday night that Mrs. Rivera had resigned, no explanation given.

What went wrong? The company isn't saying any more than to give a pat nod of ‘appreciation' by ACE USA to Mrs. Rivera - the company's top female executive - for her less than three years of service. Speculation puts Mrs. Rivera's sudden departure as being linked to evidence that she knew of questionable practices at the company that have now landed a unit, that fell under her watch, in hot water with New York Attorney General Eliot Spitzer.

Mrs. Rivera, 39, from all appearances, excelled at ACE - she was appointed president of ACE INA Holdings in March, 2002, giving her responsibility for all aspects of ACE INA's US commercial retail insurance operations. In December, 2003, she was named president and CEO of ACE USA, in a bid to “better reflect her responsibilities”.

She was widely touted as a rising star not only at ACE, but within the industry. When she joined ACE - shortly after the company's multi-billion dollar purchase of Cigna's property and casualty business - she played a role in integrating the then much smaller organisation with the wide Cigna network. She was praised by her former boss Dominic Frederico as one who would be “instrumental in fostering our rapid growth”.

And it wasn't just at ACE that Mrs. Rivera was revered.

She was also sought after in insurance circles with numerous invitation to speak on industry issues at the sector's top events. Indeed, she was slated to be one of the keynote speakers at April's Risk and Insurance Management Society Inc. meeting taking place this year in the town where she has been based, Philadelphia. But yesterday RIMS said Mrs. Rivera's participation was now something that was to be determined.

Perhaps it was Mrs. Rivera's very senior position in the company - she oversaw units that collectively employ about 4,000 people, write billions each year in premium and account for a healthy portion of the overall income for the ACE Group - that led to this end.

On top of her ACE INA and ACE USA titles, she was also, a year ago, made one of three people to oversee governance for ACE's US operations when she was appointed to the ‘Office of the Chairman of ACE INA Holdings'.

This level of seniority and oversight - with Mrs. Rivera telling underwriters at a Professional Liability Underwriting Society conference not more than two months ago “we will be seeing more accountability, oversight and transparency” - put her at a level where she needed to be beyond reproach. Expectations, when she was hired away from AIG's American Home Assurance Company were that she was destined for the highest levels at ACE.

But fast forward to October, and that is where high hopes may have begun to come undone with the contents of internal communications between her and the president of the ACE casualty risk unit being leaked to the Press.

A Wall Street Journal report said Mrs. Rivera had been warned in November 2003, in an e-mail from the president of the casualty risk unit, Geoffrey Gregory, that the way bids were being arranged “could potentially be construed as simply creating the appearance of competition”.

This means Mrs. Rivera may well have been aware of “instances of wrongdoing” (as ACE as since referred to the practices under scrutiny) long before the high-profile investigation by New York Attorney General Eliot Spitzer was launched.

Citing unnamed sources close to events, the Wall Street Journal said that rather than put a stop to the phoney bids, ACE began referring to them by an unrevealing in-house euphemism: “indication” bids.

In early November Mr. Gregory and another employee in the same unit, Patricia Abrams - who had previously pleaded guilty to a misdemeanour in connection with Mr. Spitzer's investigation - were fired by ACE for their alleged involvement in the bid-rigging scandal.

Three unnamed employees were also suspended by ACE in connection with an “ongoing investigation into improper business practices”.

Now, ACE isn't saying if Mrs. Rivera is the sixth casualty from the Spitzer investigation.

CEO Evan Greenberg has only allowed, in an open letter to employees put up on the company's website this week, that the company's own investigation is still underway - with evidence that the problems were contained within that one unit - and should be concluded by the end of February.

What is clear is that the resignation of Mrs. Rivera - who admitted in a 2003 Risk & Insurance interview, to being a “hard driver” who demanded the best of herself (reportedly passing actuarial exams at the same time of her graduation with a Mathematical Science degree from Villanova University), and those under her - is the fall, or at least set back, of one of the industry's bright lights.