"Security concerns" led to the evacuation of two planes immediately after they landed at Seattle-Tacoma International Airport on Sunday, a day after two F-16 fighter jets escorted a Southwest Airlines Flight and Delta Flight to Atlanta's airport after they received bomb threats, the Associated Press reported.

Passengers from a JetBlue flight from Long Beach, Calif., and a regional SkyWest jet from Phoenix were immediately escorted to their gates from the airport's third runway after their planes arrived on late Sunday afternoon, Airport spokesman Perry Cooper said, adding that the matter was under investigation and that he did not have further details.

Another flight, traveling from Los Angeles to Orlando, was also diverted to Dallas on the same day due to "a security concern," a Delta Air Lines spokesman said. Passengers had to be evacuated for authorities to search the Boeing 737-900, which was eventually continued by Sunday evening, Spokesman Morgan Durrant said.

Although online threats had been issued against the JetBlue and Delta flights, authorities were not able to locate any actual threat, said Laura Eimiller, a spokeswoman for the FBI in Los Angeles.

But since all three incidents were reported around the same time, investigation agents will determine whether the threats were related, she added.

The flight disruptions come a day after bomb threats targeted two jets bound for Atlanta, prompting F-16 fighter jets to escort the planes. The threats posted on Twitter targeted Southwest Airlines Flight 2492, which arrived at Atlanta from Milwaukee; and Delta Air Lines Flight 1156, which arrived from Portland, Ore., according to CNN.

"I have a bomb on one of your planes, but I forgot which one when I left the airport," the message read."Can you help me find it?"

The person said he/she smuggled the bomb through the back entrance at the airport and placed the bomb on the back of a plane with the help from a friend in airport security. The person ended the series of threats by tweeting that he/she would attack more planes if anything happened to him or her. 

Meanwhile, it remains unclear if any of threats were connected.