In the trial now underway in Manhattan Federal Court, insurers are arguing that they are bound to the Willis Property form, which specifically defines "occurrence" and would limit the Sept. 11, 2001, claim to one event of $3.55 billion.

Mr. Silverstein is contending that the limits should be determined by a Travelers form, which offers no such definition and could be interpreted to support his argument that when terrorists used a pair of jet liners to destroy the Twin Towers it constituted two insured events.

Jurors must decide whether the insurers represented in court are bound by the WilProp form or the one offered by Travelers. If jurors find that one or more of the insurers represented are not bound by WilProp and its specifically defined "occurrence," then those carriers will join Travelers in another, separate trial generally to determine whether the Twin Tower attack constitutes one or two occurrences for the purpose of insurance claims.

According to Mr. Dreyer, Mr. Silverstein's insurance program consisted of a primary and 11 excess layers in which more than 20 insurers and Lloyd's syndicates participated. Mr. Dreyer noted that the largest participant was Swiss Reinsurance Co., with a share of about 25 percent: "Therefore, a verdict against the insurers is not, in itself, a trigger for further downgrades."

S&P said its rating concerns are also diluted by the lawsuit's duration, pointing out that the jury trial, which began last week, is part of a protracted legal process that has taken more than two years to start trial proceedings.

Additionally, whether the WTC event is deemed to be one event or two, it will be "a relatively small part of the overall liability picture spawned by Sept. 11, 2001," according to the New York-based rating agency.

S&P cited research from PricewaterhouseCoopers that found other Trade Center liability issues include unresolved business-interruption claims against insurers that exceeded $8 billion at mid-year 2003.

Only 70 families with claims from the WTC event opted not to receive money from a federal victims' compensation fund by a December 2003 deadline and thus remain free to seek redress against corporations, agencies and governments. S&P said it sees this limited number of families eligible for such compensation as "a positive for the insurance industry."

"Although the payouts from the WTC attack are significant, the insurance industry has learned how to employ better modeling techniques and to spread risk more efficiently. This case proves that point," Mr. Dreyer said.