“It is hard for us, without being flippant, to even see a scenario within any kind of realm of reason that would see us losing one dollar in any of those transactions.”

— Joseph J. Cassano, a former A.I.G. executive, August 2007

Two weeks ago, the nation’s most powerful regulators and bankers huddled in the Lower Manhattan fortress that is the Federal Reserve Bank of New York, desperately trying to stave off disaster.

As the group, led by Treasury Secretary Henry M. Paulson Jr., pondered the collapse of one of America’s oldest investment banks, Lehman Brothers, a more dangerous threat emerged: American International Group, the world’s largest insurer, was teetering. A.I.G. needed billions of dollars to right itself and had suddenly begged for help.

One of the Wall Street chief executives participating in the meeting was Lloyd C. Blankfein of Goldman Sachs, Mr. Paulson’s former firm. Mr. Blankfein had particular reason for concern.