No other airlines at Sea-Tac were affected by the problem, airport spokesman Perry Cooper said.
Passengers who had their travel disrupted were told they could quickly rebook without a change 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 Chicago-Milwaukee cable was cut accidentally due to some kind of work or maintenance, Davis said, adding she did not know who was doing the work.
The second cut involved an aerial cable that runs along power lines. It wasn't clear late Monday how that got cut, Davis said.
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, according to its website.
Alaska Airlines and Horizon Air are owned by the Alaska Air Group.