Those at the Fed who have contended that Lehman was insolvent have never provided any basis for that conclusion, other than references to the estimates of Wall Street firms and other anecdotal evidence. The Financial Crisis Inquiry Commission asked for such evidence several times, but the Fed never provided it. The members of the New York Fed teams said that they did not prepare a formal, written report, and that no one asked them for any notes or work papers or asked them to elaborate on their findings. Scott G. Alvarez, the Fed’s general counsel, told the commission that there was “no time” that weekend for a written analysis.

‘A Lack of Legal Authority’

Phil Angelides, the crisis commission’s chairman, said no one ever mentioned the New York Fed analysis during his hearings. “If in fact the analysis existed and was independent, it would have been in everyone’s interest to have that out, even if it were in the form of notes,” Mr. Angelides said in an interview. He added, “If you look at the record, there is no legal stopper,” meaning a legal barrier.

So why, then, was Lehman allowed to die?

Mr. Paulson has said that politics did not enter into the decision. But he had endured months of criticism for bailing out Bear Stearns in March 2008, and the outcry only intensified after the Treasury provided support to the mortgage finance giants, Fannie Mae and Freddie Mac, in the first week of September. During a conference call on the Thursday before Lehman’s collapse, Mr. Paulson declared to Mr. Bernanke, Mr. Geithner and other regulators that he would not use public money to rescue Lehman, saying he did not want to be known as “Mr. Bailout.”

In written testimony before Congress that September, Mr. Bernanke made no mention of any legal constraint. Instead, he said, “We judged that investors and counterparties had had time to take precautionary measures.”

It was only on Oct. 7, after early praise for the decision to let Lehman fail had turned into a wave of criticism, that anyone mentioned the legal argument. In a speech that day, Mr. Bernanke said, “Neither the Treasury nor the Federal Reserve had the authority to commit public money in that way.” Mr. Paulson first mentioned the claim a week later. In an interview, Mr. Bernanke said, “We made a deliberative decision to be very cautious about publicizing our inability to save Lehman out of concern that it would further worsen the market panic.” Mr. Paulson made the same point. Mr. Bernanke was emphatic before the Financial Crisis Inquiry Commission in 2009: “I will maintain to my deathbed that we made every effort to save Lehman, but we were just unable to do so because of a lack of legal authority.”

Mr. Bernanke and others have said that a Fed lifeline to Lehman might not have stopped the run on the firm. But others have said the point of Rule 13-3 was exactly that — to stop such panics.

“Of course the Fed can stop a run,” said Mr. Blinder, the economist. “That’s what it’s all about.”

Scholars are still struggling with the claim that the Fed could not rescue Lehman but was nonetheless able to save Bear Stearns and A.I.G.

What is clear to Mr. Blinder, he says, is that the decision was a formula for panic.

“The inconsistency was the biggest problem,” Mr. Blinder said. “The Lehman decision abruptly and surprisingly tore the perceived rule book into pieces and tossed it out the window.”