A written bomb threat was discovered on a flight from Seattle to Chicago, prompting aviation officials to temporarily close Midway Airport.
Southwest Airlines' Flight 202 landed without incident Thursday but the plane was not allowed to approach the terminal, authorities said.
Instead, its 129 passengers and five crew members were taken by bus to a secure area where they met with investigators, said Wendy Abrams, a spokeswoman for the city's aviation department.
Beth Harbin, spokeswoman for Dallas-based Southwest Airlines, said the bomb threat was in a written note found aboard the plane during the landing process.
The Federal Aviation Administration shut down all air traffic at the airport at about 1:15 p.m. for more than 20 minutes. Abrams said the closure caused only "minimal delays."
The Chicago Police Department and the FBI's terrorism task force searched the plane and its luggage but no bomb had been found as of 5 p.m., said FBI spokesman Frank Bochte.
Investigators were trying to determine where the note came from, including whether it was left on the plane by a passenger on a previous flight, officials said.
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A passenger on the plane discovered a threat written on two table trays on the plane.
Authorities had believed a 911 call came from a passenger aboard the plane, but the FBI had determined the call did not originate there.
A passenger found a note scrawled on two tray tables that a bomb was on board.
Southwest Flight 117 left for Phoenix with 120 passengers at about 4:40 p.m. when a call came from the Phoenix area saying there was a bomb on the plane.