Thrifty U.S. consumers carefully picked over Black Friday discounts during the four-day weekend, leaving retailers with less money in their registers and a lukewarm kickoff to the holiday season.

Though the number of shoppers grew over the weekend, the average amount they spent fell 3.5 percent to $289.19, including both online and offline purchases, the National Retail Federation found. Consumers shelled out $299.60 in 2015, according to the trade group, which bases its figures on surveys conducted by Prosper Insights & Analytics.

The weekend was characterized by heavy markdowns and a shift toward e-commerce, meaning there were smaller crowds at the mall. Forty-four percent of consumers did their shopping online, compared with 40 percent at brick-and-mortar stores. In either case, many of them were looking for just one thing: a good deal.

โ€œOver one-third of shoppers said 100 percent of their purchases were on sale,โ€ NRF Chief Executive Officer Matthew Shay said in a statement. Thatโ€™s a jump from last year, setting an ominous tone for the rest of the holiday season โ€” retailersโ€™ biggest sales period of the year.

While the total number of Black Friday weekend shoppers grew 2 percent to 154 million, their refusal to spend as much could renew concerns about the sluggish retail economy. Companies have been trying to bounce back from a polarizing election season that they said prevented consumers from making purchases. They were hoping a strong start to the holiday shopping season would give a boost to the industry.

But retailers also are grappling with a longer-term shift toward more thriftiness โ€” aided by technology.

โ€œThereโ€™s a confluence of events here,โ€ Shay said on a conference call on Sunday. โ€œSince the recession, weโ€™ve had much more cost-conscious consumers. In the last six to seven years, people have been much more deliberate about the purchases theyโ€™ve made. And that coincided with the ability to access information in a very transparent way because of the existence of smartphones and technology-enabled pricing.โ€

Moreover, e-commerce has reduced the need to buy gifts on Black Friday weekend itself. Online sellers are doing more promotions either before or after the four-day stretch. EBay Inc., for instance, urged Thanksgiving travelers to buy things on their phones while in transit, dubbing the event โ€œMobile Wednesday.โ€

Black Friday weekend is followed by Cyber Monday, a push for e-commerce shoppers. More than 122 million consumers plan to take part, up from 121 million last year, the NRF said.

Brick-and-mortar chains have become more aggressive in chasing online customers. Wal-Mart Stores Inc. kicked off its โ€œCyber Weekโ€ promotions on Friday, and Target Corp. has been offering 15 percent off all purchases โ€” either in stores or online โ€” Sunday and Monday.

Brick-and-mortar store visits declined 1 percent during this yearโ€™s Thanksgiving and Black Friday, compared with those same days in 2015, research firm ShopperTrak found.

The discounts squeeze profit margins, but thereโ€™s often no alternative, Shay said. And in many cases, retailers have been carefully planning for the markdowns as a means as of keeping their inventory low, he said.

That decreases the amount they have to spend hanging on to excess stock.

โ€œIn a perfect world, everyone would sell everything at full price,โ€ Shay said. โ€œBut as consumers, and as buyers, all of us would like to get a deal on things weโ€™d like to buy.โ€

Spending during the overall season โ€” November and December โ€” is expected to grow 3.6 percent to $655.8 billion, the NRF estimates.

โ€œThe consumer has some ground to make up after a sluggish start to November,โ€ Matthew Fassler, an analyst at Goldman Sachs, said in a report.

Some consumers are waiting longer to start their holiday shopping, which puts pressure on retailers to keep up the promotions. Almost a quarter of consumers surveyed by Prosper Insights hadnโ€™t purchased anything on their holiday lists yet, a higher percentage than last year.

โ€œDecember might even be a little more important this year than last year,โ€ said Pam Goodfellow, the principal analyst for Prosper Insights.