(CNN) A backpack emitting smoke aboard a stationary Southwest Airlines flight on Monday drew a response from the bomb squad at Chicago's Midway International Airport.

Southwest said that Flight 285 to Austin, Texas, was in the process of boarding when smoke began showing around a customer's backpack. Flight attendants placed the backpack in a fire-resistant bag and asked the customers to leave the plane, the airline said.

The flameproof bag was moved to the jetway, where Chicago police's bomb squad and arson unit responded.

Chicago Fire Department spokesman Larry Langford told CNN that the passenger carrying the backpack heard a pop and then saw smoke coming from his backpack. Only about 12 rows had been boarded at the time of the incident, and the seat the bag was sitting on did not ignite, Langford said.

There were no reported injuries or requests for medical attention, Southwest Airlines said.

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