Southwest Airlines flights across the country were held up Wednesday while the airline worked to fix technology problems.
Southwest began having intermittent problems with several systems after an outage.
"We are now managing flight delays across our system, with a temporary ground stop in place for those flights that have not left the gate," spokesman Brad Hawkins said in an emailed statement.
For about three hours, visitors to Southwest.com couldn't buy tickets, check in for flights, or check their flight's status. The site appeared to be working again by late afternoon.
Hawkins said that systems were gradually coming back, but that it might take time before the airline could resume normal operations.
Anxious customers tweeted to Southwest that they could not check in for flights.
Leah Boyd and her husband, Matt, were flying to Providence, Rhode Island, but were held up at the Baltimore airport for three hours by mechanical issues with two different planes.
They finally boarded a plane, but after sitting at the gate for nearly an hour passengers were asked to exit because of the technology outage, Boyd said. Then the pilots reached the end of their shifts, so passengers waited for a replacement crew.
"I've never seen so many people in the terminal," she said. "If the flight is canceled we'll probably just drive tomorrow, because all these people are going to be flying standby."
By about 3 p.m. Pacific time, Southwest had canceled 16 flights, more than any other U.S. carrier, and delayed 245 others, according to tracking service FlightAware.com.
Airlines have sprawling, overlapping and complicated technology systems, and even brief outages can cause thousands of passengers to be stranded for hours.
Last October, an outage caused about 800 Southwest flights to be delayed and forced employees to issue tickets and boarding passes by hand. The airline blamed a software application, and it recovered in about a day. United Airlines and American Airlines both had computer problems last summer but fixed the problems within a day.
Southwest Airlines Co. carries more passengers within the United States than any airline. However, it is far smaller than American, Delta and United when international traffic is included.