SEATAC, Wash. — Increased summer travel is responsible for delays at Seattle-Tacoma International Airport security lines, the airport and Transportation Security Administration said.

About 150 passengers have missed flight on Alaska Airlines since Sunday because of waits of more than an hour in security lines, airline spokeswoman Marianne Lindsey said Thursday.

"Summer travel season is on," she said.

The airline is sending text messages to travelers advising them to prepare for an hour at security checkpoints, she said.

The worst times are between 4 p.m. and 9 p.m. when Alaska has 60 flights departing. Alaska is the largest single carrier at Sea-Tac.

The airport advises passengers to arrive two hours early, said spokeswoman Christina Faine.

SeaTac averages about 100,000 passengers a day from June through August, compared to about 85,000 off-season.

"The length of lines at security checkpoints varies by airport and travel season. A fluctuation in wait times can be seen especially during peak travel such as summer break," said regional Transportation Security Administration spokeswoman Lorie Dankers.

Wait times Thursday morning were around 9 minutes, she said.

The delays are frustrating for everyone involved.

"We let them know we're going to have full loads," Lindsey said. "Truthfully, they should have been prepared for the influx. The loads we're seeing are not uncommon for this time of year. This is our busy season."