DALLAS – Southwest Airlines Co.’s website is slowing to a crawl, and the airline blames heavy response to a sale on fall travel.

Visitors to southwest.com late Wednesday afternoon saw a “system alert” warning that “you may experience difficulty” on the site. When one visitor tried to book a trip, there was no immediate response from the site, and eventually the screen flipped to a “Gateway Timeout.”

Southwest spokeswoman Brandy King said the airline added capacity to the website in anticipation of high demand from a 3-day sale that began Tuesday, but the response was bigger than expected. King said workers were trying to restore the website’s functions. She said customers could try later or call the airline for help.

The sale included prices as low as $49 one-way on short trips to $149 each way for long trips, and covered travel between Aug. 25 and Dec. 16. Like most airline sales, there were many conditions including blackout dates around Labor Day and Thanksgiving, and Southwest didn’t say how many seats would be available at the sale prices.

American Airlines and United Airlines confirmed that they matched Southwest’s fares on routes where they compete.

Rick Seaney, CEO of FareCompare.com, said Delta, Alaska, Virgin America and Frontier also had fares matching Southwest on overlapping routes, although he added that it was “hard to tell who matched and who initiated” because there were so many new fares filed on Tuesday. The first week in June is a perennial time for fall-travel sales, he said.