The Sept. 11, 2001, terrorist attacks, the worst insured catastrophe in American history, were not costly enough to trigger failures of any U.S. insurance companies, according to a university researcher's study.

"There was this major fear that our insurance industry would be wiped out by these acts simply because of the enormous payouts," said John Fitzgerald, a finance and insurance professor at Ball State University in Muncie, Ind., who expects to finalize the study in the coming weeks. "However, the only business failures were actually normal exits from the industry."

While initial estimates of Sept. 11 losses ranged as high as $70 billion in the weeks after the attacks, actual damage payments have amounted to between $30 billion and $35 billion over the past three years, Mr. Fitzgerald noted.

Losses have been lower than initially expected in part because victims or their families chose to participate in the federal Sept. 11 Victims Compensation Fund rather than suing airlines, security firms, building owners and others.

In addition, while some insurers--among them Berkshire Hathaway Inc. and American International Group Inc.--suffered heavy losses, insurers generally reinsured their New York City exposures extensively, spreading losses throughout the industry. As a result, no U.S. insurer faced larger losses than it could bear, Mr. Fitzgerald concluded.

One Japanese insurer, Taisei Fire & Marine Insurance Co. Ltd., blamed its 2001 bankruptcy on huge Sept. 11 aviation losses assumed through U.S. managing general agent Fortress Re Inc., Mr. Fitzgerald noted. Non-U.S. insurers were not included in the study, he said.

Fears of ongoing uninsured terrorism exposure have also proven overblown, he suggested.

While some businesses worried soon after Sept. 11 that the high cost of terrorism coverage would force them to close--and while the insurance industry has introduced 47 different terrorism exclusions to various types of policies--"American businesses have adapted to the changing climate," Mr. Fitzgerald found. "Today, terrorism is simply thought of as a natural part of doing business. It has become a simple risk management issue."