New retail figures from the Department of Commerce paint a bleak December sales picture. So bleak some analysts are questioning the validity of the just-released figures that were delayed by several weeks due to the government shutdown.

U.S. retailers’ nonstore sales reached $76.96 billion in December on a non-adjusted basis, a 3.1% increase compared with $74.62 billion in the same month of 2018, new monthly data from the U.S. Commerce Department shows. For all of 2018, nonstore sales grew 9.6% on a non-adjusted basis to reach $678.79 billion compared with $619.22 billion for 2017. The sales estimates are preliminary figures and may be adjusted.

The December nonstore sales growth is significantly lower than the year-over-year monthly increase in November, which was 12.1%. Full-year nonstore sales also represent a slowdown. In 2017, nonstore sales grew 10.4% on a non-adjusted basis.

Nonstore sales mainly take place online but also include other channels, such as mail and telephone orders, door-to-door sales and sales through vending machines.

Total retail sales reached $372.61 billion in December on a non-adjusted basis, up a meager 0.9% from $369.19 a year earlier. For 2018, total retail sales reached $3.67 trillion, up 4.3% $3.52 trillion a year earlier. These figures factor out most goods not normally purchased online, including food services, gasoline and automobiles.

The Commerce Department only reports ecommerce sales on a quarterly basis. The agency will release its fourth quarter and full-year ecommerce sales results on March 13.

The fact that December nonstore sales grew so slowly—just 3.1% on an unadjusted year-over-year basis—even as Amazon.com Inc. alone saw its net sales in North America grow about 18% in the fourth quarter, has some industry observers questioning the validity of the data.

Jim O’Sullivan of High Frequency Economics said the figures were so much weaker than expected “that the data lose credibility,” while Stephen Stanley of Amherst Pierpont Securities said the report “seems seriously out of whack” given mostly upbeat comments from retailers about the Christmas season.

The report was delayed about a month due to the partial federal closure. “I’m actually wondering whether the government shutdown created issues for them in terms of data collection and quality,” said Neil Dutta, head of economics at Renaissance Macro Research LLC.

The Commerce Department’s Census Bureau said in Thursday’s report that while data collection was delayed by the shutdown, “processing and data quality were monitored throughout and response rates were at or above normal levels for this release.”

“These numbers are horrible,” said Ward McCarthy, chief financial economist at Jefferies LLC. “It appears to contrast quite sharply with reports of Christmastime sales that were generally seen as quite healthy.”

Bloomberg News contributed to this article.