Even financial professionals were grasping for answers. Many were selling — and many were buying as fast as they could. Each was making a bet on whether last week was a hiccup, or the start of something worse.

9:45 p.m. Friday, Aug. 5: Waterbury Center, Vt.

As she watched her beloved Red Sox battle the Yankees, Karissa McDonough opened her laptop to check e-mail.

Jumping off the screen was a subject line from a news alert at 9:03 p.m.: “U.S. Long-Term Debt Downgraded by Standard Poor’s.”

She stared at her screen in disbelief. Ms. McDonough, the chief bond strategist at People’s United Bank in Connecticut, had spent her working life analyzing the debt of companies and governments. This news, however, was surreal. This happened in other countries. Not here.

“We were just downgraded!” she exclaimed to her husband.

“Who’s we?”

“The United States!”

She had to react swiftly. Clients might panic. Money managers at People’s United would need to know what to say. She began tapping on her laptop. At 11:02 p.m. she sent an e-mail to more than 100 colleagues. The downgrade, she told them, was “historic but not catastrophic.”

Ms. McDonough went on: “The U.S. Treasury market remains the safest, largest, most liquid investable asset class in the world.” Governments like China’s, she said, would be hard-pressed to find a better alternative.

She collapsed into bed. She had a sinking feeling that her vacation on Cape Cod, scheduled for the coming week, was about to be downgraded, too.

Noon Saturday, Aug. 6: Pasadena, Calif.

Stephen Walsh, the chief investment officer of Western Asset, had been expecting the e-mail all day.

“How do you think the markets will respond on Monday?” a worried official from the United States Treasury asked him.

A few weeks earlier, Western Asset, one of the nation’s largest bond fund managers, had asked Standard Poor’s for a private briefing about America’s credit rating. S. P. had already announced that there was an even chance that the nation’s AAA rating might be lowered.

The S. P. analysts were prohibited from saying more. But when they met with Western Asset, it was clear that the odds had worsened. “You could tell a lot by their body language, how carefully they answered certain questions,” said one of the people in those meetings. The analysts didn’t say anything specific, but they didn’t have to. Afterward, Mr. Walsh took his top lieutenants into his office, one by one. A downgrade is inevitable, he told them. We need a plan.

Now, it had happened. The previous night, the once unthinkable had occurred. The Treasury official from Washington wanted to know how Mr. Walsh — a man who indirectly controls half a trillion dollars in assets — thought the markets would respond. The Treasury was planning to sell billions of dollars of new bonds in the coming week. If buyers didn’t materialize at the auctions, it could be disastrous for the government, the financial markets and the entire global economy.

”I think investors understand it’s not that big a deal,” Mr. Walsh told the Treasury official.

It was the same thing he had been saying all day to colleagues, hedge funds and investment banks seeking his advice. “U.S. Treasuries are still the most high-quality instrument out there,” he told them. People need a safe place to put their money, and if the downgrade unsettled the stock market, as he suspected it would, investors would flock to safe investments like Treasuries, regardless of S. P.’s opinions.

“World investors didn’t downgrade U.S. debt; S. P. did,” he told one caller. “It’s just one opinion.”

When he hung up, his wife turned to him.

“What if you’re wrong?” she asked.

“That’s the great thing about Wall Street,” he replied. “We’ll know Monday morning if I’m a genius or an idiot.”

11:30 p.m. Sunday, Aug. 7: East 66th Street, Manhattan

It looked like a sell-off. Not a panic like the one that had struck in 2008, but something different.

Michael Warlan, who heads equity trading for Third Avenue, a mutual fund company in Manhattan, sat in his home office on the Upper East Side, fielding calls and sending instant messages to brokers around the globe.

Amid the turbulence, he smelled opportunity.

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