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.

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

Although it was not widely known, Goldman, a Wall Street stalwart that had seemed immune to its rivals’ woes, was A.I.G.’s largest trading partner, according to six people close to the insurer who requested anonymity because of confidentiality agreements. A collapse of the insurer threatened to leave a hole of as much as $20 billion in Goldman’s side, several of these people said.

Days later, federal officials, who had let Lehman die and initially balked at tossing a lifeline to A.I.G., ended up bailing out the insurer for $85 billion.

Their message was simple: Lehman was expendable. But if A.I.G. unspooled, so could some of the mightiest enterprises in the world.

¶ 10:48 PM