The numbers: Sales at retailers fizzled in December and posted the biggest decline in nine years in a worrisome sign for the U.S. economy, according to a long-delayed government report.

Retail sales sank 1.2% in December, the U.S. Census Bureau said Thursday. It’s the largest drop since September 2009, a few months after the end of the Great Recession.

Economists polled by MarketWatch expected sales to be flat.

The disappointing drop in sales battered stocks in Thursday trades, but many economists were skeptical that sales were quite the disaster the report seemed to indicate.

Read:The stunning drop in retail sales has Wall Street’s economists skeptical

What happened: Retailers faced plenty of headwinds in December, including a stock-market meltdown, sudden talk of recession, the start of the partial government shutdown and a bout of unusually poor weather.

Yet the large decline in sales appears to go beyond that and offers more proof the economy slowed toward the end of 2018. Sales fell in every retail category except auto dealers and home centers.

Sales fell the steepest, 5.1%, at gas stations, but that was not unexpected. Gasoline prices have been falling since last fall.

What’s was surprising was a 3.9% reported decline in sales at internet sellers. That would mark the sharpest drop since November 2008 — the middle of the last recession. Yet by all industry accounts, online merchants led by Amazon AMZN, +5.69% and eBay EBAY, +1.55% reaped big sales gains.

Less surprisingly, sales tumbled 3.3% at department stores that have been losing ground for years to mainly internet-based competitors. Traditional brick-and-mortar chains such as Macy’s M, -0.63% , Kohl’s and Nordstrom posted disappointing sales in December.

Sales also fell at bars, restaurants, apparel stores, grocers, home furnishers, pharmacies and outlets that sell hobby items such as books and sporting goods.

Sales rose 1% at auto dealers while home-center sales edged up 0.3%.

Big picture: The sales slump in December will weigh down the government’s official scorecard for the economy known as gross domestic product. Economists had estimated GDP would slow to 2.7% in the final three months of the year, but now that estimate is being taken down even further.

What does it mean for the economy in 2019? Hard to say. The stock market tanked in December and talk of recession briefly became all the rage, and that may have hurt sales.

Yet the stock market rebounded in January, consumer confidence remains high and companies continued to hire at a strong clip. The economy should avoid a recession as long as the labor market remains health, but growth in 2019 is unlikely to match the strong performance of 2018.

What they’re saying: “These data are so wild that we have to expect hefty upward revisions, but if they stand, they are very unlikely to be representative of the trend over the next few months. The consumer is no longer enjoying tax cuts or falling gas prices, but that’s no reason to expect a rollover,” said Ian Shepherdson, chief economist at Pantheon Macroeconomics.

“It’s a puzzle. Strong job gains, wage growth, and the drop in gasoline sales should be very supportive of consumer spending growth,” added Scott Brown of Raymond James.

Market reaction: The Dow Jones Industrial Average DJIA, +0.51% and S&P 500 SPX, +1.05% traded lower in early action after both the weak retail report and surprising increase in new jobless claims.