DALLAS » A massive slowdown on the Southwest Airlines website stretched through a second day on Thursday, leaving many customers unable to book flights during a big fare sale.

Southwest blamed the meltdown on heavy traffic from the sale. The carrier extended it by one day, to midnight on Friday night, to give frustrated customers more time to buy tickets.

On Thursday afternoon, some customers trying to book flights online got a message telling them that part of the website "is undergoing maintenance and is currently unavailable." The message directed people to call Southwest’s toll-free number, but a reporter who did that got busy signals.

Southwest officials said the 800 number was working but call volume was high. At times, customers had trouble checking in for flights online, they said.

Executive Vice President Bob Jordan said the problems were limited to southwest.com and the company’s mobile app and did not affect other systems, such as reservations.

"We plan for these (sales) literally months in advance," Jordan said in an interview. "In this case we just have an unexpected issue … the site has just been a little overwhelmed."

Jordan said Thursday evening that technicians were making progress, but he declined to predict when the website might return to normal.

Airlines are highly dependent on technology systems for jobs ranging from selling tickets online to setting flight plans. Earlier this week, United Airlines suffered a short outage of its flight-dispatch system that led to the cancellation of 150 flights.

Southwest’s troubles have lasted longer than most. Southwest could be particularly vulnerable this time because it sells a higher percentage of tickets on its own website instead of through online travel agencies. That approach has often been seen as a strength because it keeps distribution costs down.

Several other airlines said they matched Southwest prices on routes where they compete, so it is likely that some people who intended to book on Southwest this week wound up buying a ticket on another airline.

"This is a catastrophic technology failure when your primary sales channel is unavailable for more than a day," said Henry Harteveldt, a travel-industry analyst for Atmosphere Research Group. "There will be a lingering, negative effect in terms of (passengers’) willingness to consider flying on Southwest."

Southwest has run many similar sales without the website suffering such a lengthy breakdown, he noted.

Ty McMahan, director of content at online-retail research firm StellaService, said a website crash lasting more than 24 hours was uncommon. Last year on Black Friday, the huge shopping day after Thanksgiving, Best Buy’s site went down — it blamed a surge of traffic from mobile devices — and Nike had trouble processing orders. Both recovered within hours.

Customers will give companies a break as long as they get their cheap deal, McMahan said. "What is at risk is that everyday customer who can easily turn to a competitor for the same product," he said.

The Southwest sale featured prices as low as $49 one-way on some short trips and $149 each way on longer flights between Aug. 25 and Dec. 16. There were blackout dates on Fridays and Sundays and around Labor Day and Thanksgiving. Southwest didn’t say how many seats were available at the sale fares.

Jordan said sales volumes returned whenever technicians could improve the website’s performance. "What we see is our loyal customers really trying to get through."