The unofficial start of summer over the Memorial Day weekend offers a troubling glimpse of what lies ahead for travelers during the peak vacation season.

U.S. airlines canceled more than 2,800 flights from Thursday through Monday, or about 2 percent of their schedules, according to tracking service FlightAware.

Delta Air Lines, usually among the top performers, had the worst record among major carriers with more than 800 canceled flights over the five-day span.

“This was a chance for airlines to show that last summer’s delays would not be repeated this summer, and yet, it was not to be,” said Helane Becker, an analyst for banking firm Cowen, who blamed the disruptions on bad weather, air traffic control delays, airline crew members calling in sick, and long security lines at some airports.

“We expect a busy summer, and are concerned about the industry’s ability to handle the demand,” Becker said.

The good news was that cancellations were down sharply on Tuesday. FlightAware reported only about 60 by early afternoon on the East Coast.

Various forecasts of high numbers of travelers over the weekend proved to be accurate. The Transportation Security Administration reported screening more than 11 million people at airport checkpoints from Thursday through Monday.

That was down 9 percent from the same days in 2019, but an increase of almost 25 percent over last year. Crowds of just under 2.4 million on both Thursday and Friday nearly matched the pandemic high set on the Sunday after Thanksgiving last year.

Delta, when asked to comment Tuesday about its weekend troubles, pointed to a statement it issued last week. The Atlanta airline said it was being challenged by several factors including rising COVID-19 cases among workers, and it trimmed its July and August schedules in an effort to improve reliability.