When the stock market goes down, people saving for retirement are quick to worry about their investments. Here's why 401(k)s and IRAs are better left alone. (Jayne Orenstein,Daron Taylor/The Washington Post)

A global stock market sell-off that once appeared likely to only dent investor accounts turned into a larger retreat Friday as concerns grew over the state of China’s economy and the free fall in oil prices.

The major U.S. markets tumbled more than 2 percent, leaving them near their lowest levels in more than a year. Hopes that U.S. investors might be spared the worst of the pullback were replaced by predictions that the losses are likely to get worse. The declines have already wiped out more than $1 trillion in wealth this year from the shares of 500 of the largest U.S. companies.

The rapid turnaround caught some investors by surprise. For many, a historic five-year run of stock gains had erased their losses from the last big market meltdown during the Great Recession, and many moved to protect their portfolios.

“We have a generation of investors who have witnessed corrections that have morphed into crisis,” said Jason Katz, a senior portfolio manager with UBS Wealth Management.

Financial advisers have been rushing to calm worries. Some Merrill Lynch advisers said they have begun reaching out weekly to clients by email and phone instead of monthly. Chicago-based BMO Private Bank began preparing its clients for the market turbulence last summer, suggesting they sell some stock, buy more bonds and hold more assets in cash.

Many, such as UBS Wealth Management, are warning clients to temper their expectations. “We’re clearly in a tumultuous time where we’re expecting further volatility where things might get a little worse before it gets better,” Katz said.

Stocks were led astray Friday by crude-oil prices that fell below $30 a barrel and growing concerns that the U.S. economy may not be as strong as it appears. U.S. consumers curbed their spending in December, handing retailers a lackluster finish to a weak year, according to Commerce Department data released Friday.

Much of the recent decline has been tied to a slowdown in China’s economy. The closely watched Shanghai Composite Index fell an additional 3.6 percent Friday, entering bear-market territory, or down more than 20 percent from its high-water mark.

In the United States, the Dow Jones industrial average, a barometer of 30 blue-chip stocks, and the Standard & Poor’s 500-stock index, a broader view of the market, fell an additional 3 percent this week. They are down 8 percent this year.

But more potentially concerning for Main Street investors is that the S&P 500 and the tech-heavy Nasdaq have both fallen 10 percent from the record highs they reached last year, a retreat known as a “correction.” Such a drop is easy to dismiss as a technical anomaly, but its the kind of fall that can spur investors to sell, leading to even a deeper slide, analysts said.

U.S. stocks could fall an additional 10 percent, Larry Fink, the chief executive of BlackRock, the world’s largest money management company, said in a CNBC interview Friday morning. “There’s not enough blood in the street.”

U.S. investors have entered a tricky period in which a relatively healthy economy at home may not be enough to carry markets higher, analysts said. Weakness in one part of the world, or in a single sector, can quickly spread. Concerns about the strength of the manufacturing sector in China, the world’s second-largest economy, have triggered days of sell-offs around the world. The sharp fall in crude-oil prices has sparked concerns about the health of the energy sector and businesses in general.

And yet, the U.S. economy remains relatively healthy compared with the rest of the world. Employment is strong. Auto sales have reached a new high, and the housing sector continues to rebound. The current sell-off follows five years of stock growth, and many market analysts have warned for months that a pullback was inevitable.

Stock values ballooned while the Federal Reserve kept interest rates near zero, said Jack Ablin, the chief investment officer for BMO Private Bank. But now that the Fed has begun to raise rates, investors are becoming more critical of signs of weakness on corporate balance sheets, he said.

“I view days like this as cathartic,” Ablin said Friday. “This is what we need to move forward.”

That may provide little solace to individual investors. The anxiety is most acute among retirees or those preparing to retire, analysts said. According to a 2015 Fidelity investments report, many people nearing retirement, between the ages of 50 to 54, had too much of their savings locked into the stock market.

Making things more complicated, analysts said, is that there are few alternatives. Despite the Fed’s move to raise interest rates last year, the returns an investor can make on safer investments such as bonds, for example, remain paltry, they said.

“The rank-and-file middle class here, they are just taking it on the chin,” said Michael Thompson, chairman of S&P Investment Advisory Services. “People should be very careful to have their portfolio’s risk consistent with what is suitable for their age — and what they’re willing to lose, period.”

Michael K. Farr, president of investment firm Farr, Miller & Washington, said people should take a deep breath, not panic, and keep their eye on the long horizon.

“This has been a long time coming,” he said. “The markets are going to correct, and stocks are going to find a normalized level. We know that down markets are unpleasant. These are the markets through which long-term investors earn their stripes.”

Marter reported from Washington. Thomas Heath in Washington contributed to this report.