WASHINGTON — The lowering of America’s sterling credit rating was the punctuation mark on a tumultuous week in financial markets.

Standard & Poor’s, the credit rating agency, said Friday it was dissatisfied with the plan Congress came up with earlier in the week to reduce the country’s debt.

This is the first time the nation’s credit rating has fallen below the highest level, AAA. The U.S. had held that rating since 1917. The move came just days after a gridlocked Congress finally agreed to spending cuts that would reduce the debt by more than $2 trillion.

The drop in the rating by one notch to AA-plus was telegraphed as a possibility back in April. The three main credit agencies, which also include Moody’s Investor Service and Fitch, had warned during the budget fight that if Congress did not cut spending far enough, the country faced a downgrade. Moody’s said it was keeping its AAA rating on the nation’s debt, but that it might still lower it.

One of the biggest questions after the downgrade was what impact it would have on already nervous investors. While the downgrade was not a surprise, some selling is expected when stock trading resumes Monday morning. The Dow Jones industrial average fell 699 points this week, the biggest weekly point drop since October 2008. The weak economy was the primary catalyst behind that plunge, but the debt debate and the threat of a downgrade were also factors.

“I think we will have a knee-jerk reaction on Monday,” said Jack Ablin, chief investment officer at Harris Private Bank.

But any losses might be short-lived.

“The market’s already been shaken out,” said Harvey Neiman, a portfolio manager of the Neiman Large Cap Value Fund. “It knew it was coming.”

One fear in the market has been that a downgrade would scare buyers away from U.S. debt. If that were to happen, the interest rate paid on U.S. bonds, notes and bills would have to rise to attract buyers. And that could lead to higher borrowing rates for consumers, since the rates on mortgages and other loans are pegged to the yield on Treasury securities.