Mr. Gorton's models harnessed mounds of historical data to focus on the likelihood of default, and his work may indeed prove accurate on that front. But as AIG was aware, his models didn't attempt to measure the risk of future collateral calls or write-downs, which have devastated AIG's finances.

The problem for AIG is that it didn't apply effective models for valuing the swaps and for collateral risk until the second half of 2007, long after the swaps were sold, AIG documents and investor presentations indicate. The firm left itself exposed to potentially large collateral calls because it had agreed to insure so much debt without protecting itself adequately through hedging.

The credit crisis hammered the markets for debt securities, sparking tough negotiations between AIG and its trading partners over how much more collateral AIG should have to post. Goldman Sachs Group Inc., for instance, has pried from AIG $8 billion to $9 billion, covering virtually all its exposure to AIG -- most of it before the U.S. stepped in.

Such payments continued after the government bailout. AIG already has borrowed $83.5 billion from the Federal Reserve, a little more than two-thirds of the $123 billion in taxpayer loans made available to AIG so far. In addition, AIG affiliates recently obtained from the government as much as $21 billion in short-term loans called commercial paper. Much of the $83.5 billion has been used to meet the financial obligations of the financial-products unit. If turmoil in the markets causes prices of many assets to fall further, the government might have to cough up more money to help keep AIG afloat. Cutting it off would risk renewing the market upheaval the policy makers have struggled to tame.

Mr. Gorton, the son of a Phoenix psychiatrist, took a circuitous route to academia. He studied Mandarin, considered becoming an actor and briefly drove a cab in Cleveland, where he carried a gun for protection, he later told acquaintances. Eventually, he collected multiple degrees, including a Ph.D. in economics, and joined the faculty of the Wharton School of the University of Pennsylvania.

He drove an old Volkswagen Beetle, lived in a gritty North Philadelphia row house and accumulated a vast trove of jazz records, which he would cue up at night on two turntables to keep the music coming, recalls his wife at the time, Rachel Bliss.

He was passionate about mathematics, engaging in late-night conversations with fellow teachers, says Ms. Bliss. One of his academic interests was how banks could unload risk and sell loans to investors.

In 1987, AIG launched its financial-products unit with Howard Sosin, a math expert and former Drexel Burnham Lambert executive. Among his hires were Joseph Cassano, a former Drexel colleague. After Mr. Sosin left, Mr. Gorton joined as a consultant in the late 1990s. Mr. Cassano later took over the unit.

Early on, Mr. Gorton billed AIG about $250 an hour, which likely would have netted him about $200,000 a year, says a former senior executive at the unit. Eventually, his pay was far greater; another former colleague estimates it at $1 million a year.

Mr. Gorton collected vast amounts of data and built models to forecast losses on pools of assets such as home loans and corporate bonds. Speaking to investors last December, Mr. Cassano credited Mr. Gorton with "developing the intuition" that he and another top executive had "relied on in a great deal of the modeling that we've done and the business that we've created."

 AIG began selling credit-default swaps around 1998. Mr. Gorton's work "helped convince Cassano that these things were only gold, that if anybody paid you to take on these risks, it was free money" because AIG would never have to make payments to cover actual defaults, according to the former senior executive at the unit. However, Mr. Gorton's work didn't address the potential write-downs or collateral payments to trading partners.

AIG became one of the largest sellers of credit-default-swap protection, according to a Moody's Investors Service report last week. For years, the business was extremely lucrative. In a 2006 SEC filing, AIG said none of the swap deals now causing it pain had ever experienced high enough defaults to consider the likelihood of making a payout more than "remote, even in severe recessionary market scenarios."

AIG charged its trading partners a fraction of a penny a year for every dollar of credit protection. The company realized, of course, that it might have to post collateral if the market values of the underlying securities declined. But AIG executives believed that such price moves were unlikely to occur, according to people familiar with AIG's operation.

As the debt securities created by Wall Street became more complicated, so did the swaps AIG offered. Around 2004, it began selling swaps designed to provide insurance on securities called collateralized-debt obligations, or CDOs, that were backed by securities such as mortgage bonds. Merrill Lynch & Co., then a major seller of the CDOs, was a big client.

So-called multisector CDOs, in particular, were exceptionally complex, involving more than 100 securities, each backed by multiple mortgages, auto loans or credit-card receivables. Their performance depended on tens of thousands of disparate loans whose value was hard to determine and performance difficult to systematically predict. In assessing their risk, Mr. Gorton constructed worst-case scenarios that factored in the probability of defaults on the underlying securities.

In late 2005, senior executives at the unit grew worried about loosening lending standards in the subprime-mortgage market. AIG decided to stop selling credit protection on multisector CDOs, partly due to "concerns that the model was not going to be able to handle declining underwriting standards," Mr. Gorton told investors last December.