Passengers were rushed out of two aircraft upon touchdown at Washington's Seattle-Tacoma International Airport due to an apparent security concern, an airport spokesman on Sunday.

The spokesman, Perry Cooper, said a SkyWest plane from Phoenix and a JetBlue aircraft from California's Long Beach were evacuated upon their arrival at the gates close to the runway late Sunday afternoon.

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Authorities took passengers to their gates upon the planes' descent at the airport's third runway as a precaution, Cooper said, adding a probe into the incident was underway. He declined to give more details.

An FBI spokeswoman in Los Angeles, Laura Eimiller, said threats made online zeroed in on the Delta and JetBlue flights. She ruled out solid threats to the planes besides the online messages. Federal agents undertaking the probe will determine whether there was a connection between the two airline threats.

On the same day, a Delta Air Lines representative revealed a flight to Orlando from Los Angeles had to land at Dallas late afternoon, also due to an apparent security concern.

Delta representative Morgan Durrant said flight 1061 passengers were ushered off the Boeing 737-900 so investigators can comb through aircraft. Delta completed the flight on Sunday evening.

The flight security precautions happened barely a day after bomb threats were made against two planes flying to Atlanta, prompting the U.S. military to deploy F-16 fighter jets as plane escort.

The messages sent over microblogging site Twitter mentioned flight 2492 of Southwest Airlines, which landed in Atlanta from Milwaukee; and flight 1156 of Delta Air Lines, which flew from Portland, Oregon.

It was not known if any of the threats were linked.

A user named @KingZortic sent out the tweets, which an airline employee came across at on Saturday afternoon. The tweet said: "I have a bomb on one of your planes, but I forget which one when I left the airport. Can you help me find it?"

Airline officials found the bomb threat credible enough. The tweet said the explosion device was slipped inside the Delta plane due to loose security at the plane's rear with the help of a Delta insider.

Atlanta authorities found no bomb in the Delta plane. 

TagsSeattle, Delta Airlines, JetBlue, Flight security, bomb threat, SkyWest