Sea-Tac spokesman Perry Cooper said an unknown passenger entered a secure area of the airport through an exit at 7:30 p.m.
Unable to track the person down, airport officials decided to "dump" concourses C, D and the central terminal, which means everyone inside those areas was forced to leave and go back through security. Concourses A and B remained open.
After spending 90 minutes searching for the passenger using a photo capture from a security camera, Police and TSA officials finally found him just before 9 p.m.
"The unfortunate situation at this point was that it took too long to find the individual. So once it gets to a certain point, our officers made the decision to clear multiple concourses," Cooper said.
The passenger was questioned and released. Cooper blamed the breach on a misunderstanding and said it was not a "nefarious act."
After the breach was cleared, passengers began the process of re-screening at roughly 9:20 p.m. TSA officials said it took 30 minutes to screen the passengers.
The entire ordeal created a long night for many passengers.
"I was here about an hour early and we've been waiting about an hour right now to be let out of the airport, and now we're being told to be rescreened. It should be an interesting evening," said passenger Eric Horvitz.
Nancy Trott of Alaska Airlines said 20 of the company's outbound planes and 17 of its inbound planes were delayed as a result of the breach.
TSA officials say the circumstances surrounding this incident are under review.