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The downgrade torched confidence, however, and that will hurt the economy, he said. “What the economy lacks is consumer and business confidence.”

A statement from the Fed in midafternoon initially appeared to add to the unnerving news, although it was later credited with aiding the stock rebound. The Fed said that it would hold short-term interest rates near zero for at least another two years — an unprecedented commitment to such low rates.

But the Fed did not offer any new measures to aid the economy, triggering a plunge in stocks. While economic growth this year has been “considerably slower” than expected and “downside risks” to the economy have increased, the Fed said, it must remain vigilant against inflation as well as against higher unemployment.

Mr. Chatterjee said markets were initially disappointed because they were hoping recession worries would prompt the central bank to revive a controversial Treasury-bond purchase program it ended in June. But the Fed did not comply.

A split on the Fed’s rate-setting committee, with three hawkish Fed bank presidents voting against even the small but unprecedented change the Fed made in extending its period of rock-bottom interest rates, suggests that Fed Chairman Ben S. Bernanke would have an uphill fight to push another round of major easing through.

The Fed chose to try to calm markets with “dovish language,” Mr. Chatterjee said, in part because a report showing a pickup in hiring activity on Friday gave it “enough comfort” to stay on the sidelines.

Further, while markets have lost a good deal of value in recent days, from the Fed’s perspective, nothing like the “dislocations” that threatened to bring down the financial system in 2008 have occurred, he said.

Lance Roberts, chief strategist at Streettalk Advisors, said that while the Fed chose to stand pat on Tuesday, it likely will take further action in coming weeks if the economy continues to deteriorate, with the annual growth rate dropping below 1 percent or unemployment rising significantly above 9 percent.

“We have to remember that in the ‘30s, the government stood on the sideline as federal stimulus got drained out of the economy, resulting in the double-dip sequel to the Great Depression,” he said. Mr. Bernanke, who is an expert on the Great Depression, won’t want a repeat of that.