Two planes were evacuated upon arrival and a third plane was diverted on Sunday, airport officials said.
The two jets were evacuated after arrival at Seattle-Tacoma International Airport due to a "security concern, Airport Spokesman Perry Cooper said.
A separate flight from Los Angeles to Orlando was diverted to Dallas Sunday afternoon also due to "a security concern," Delta Airlines Spokesman Morgan Durrant said.
The flights evacuated at the Seattle airport were a JetBlue plane from Long Beach, California, and a regional SkyWest jet from Phoenix.
Passengers were evacuated onto the airport's third runway and then bused to their gates, Cooper said. The spokesman also said that the investigation was still underway and he did not have further details.
A passenger who was evacuated at Seattle airport tweeted a series of photos from the tarmac:
All passengers from the flight diverted to Dallas were safely evacuated, the airline's spokesman said.
Durrant said the Delta flight's passengers were safely evacuated and he would defer to authorities before releasing more information.
The flight was allowed to continue on its journey to Orlando after getting the go ahead, Reuters reported.
Sunday's events come a day after bomb threats targeted two flights bound for Atlanta, prompting fighter jets to escort the planes.
The FBI confirmed that no bombs were found on the planes after they landed at the international airport.