SEATTLE — Alaska Airlines operations slowly were returning to normal Monday afternoon after a fiber-optic outage shut down its ticketing system for five hours, cancelling flights in Seattle and Los Angeles and causing delays across the airline’s 64-airport network.

The problems started shortly before 8 a.m. when computers the airline uses to check in passengers went down, forcing employees to board travelers manually — in some instances sticking handwritten flight numbers to computer monitors.

The biggest delays were at Seattle’s Sea-Tac Airport, where about half the flights are Alaska Airlines or its regional airline Horizon Air.

Frustration grew along with the lines of passengers waiting to check in, though some travelers simply sat or lay down and waited. Lines stretched out the door in the Alaska Airlines part of the terminal.

Airline representatives apologized and said technicians were working as fast as possible to restore the connection to the company’s Sabre reservation system.

No other airlines at Sea-Tac were affected by the problem, airport spokesman Perry Cooper said.

The outage forced a number of cancellations, including three flights out of Sea-Tac and three flights to Sea-Tac from Las Vegas, Ontario, Calif., and Orange County, Calif. At least two Los Angeles International Airport flights also were canceled.

Other locations that experienced delays included Alaska and Portland, Ore., Alaska Airlines spokeswoman Marianne Lindsey said.

All passengers who had their travel disrupted were told they could rebook without a rescheduling fee.

The problem was caused by a combination of two cut cables in Sprint’s fiber-optic network. One occurred at a construction site along railroad tracks between Chicago and Milwaukee and the other was somewhere between Portland and Seattle, said Sprint spokeswoman Crystal Davis in Reston, Va.

“Typically if there’s just one cut, traffic reroutes automatically,” Davis said. “Because there were two cuts within hours of each other, it caused this disruption.”

The Seattle-based Alaska Airlines is the seventh-largest U.S. airline based on passenger traffic and is the dominant U.S. West Coast air carrier. It has an average of 436 flights a day at 64 destinations, including airports in Mexico and Canada.

Alaska Airlines and Horizon Air are owned by the Alaska Air Group.