Story highlights Flight attendants: Two faces and the words "bye bye" were painted on an airliner

United Airlines says safety experts examined the plane and found no problems

(CNN) Thirteen former United Airlines flight attendants say they were fired for refusing to work a commercial flight from San Francisco to Hong Kong after the airline discovered "threatening" words and "menacing" images drawn on the tail cone of the Boeing 747.

The aircraft was being prepared for takeoff when flight attendants became aware oil from the aircraft's auxiliary engine was used to paint the words "BYE BYE" in 6-inch-high letters above two faces, "one smiling and the other with a more troubling devilish expression," according to a federal complaint filed Wednesday with the Department of Labor.

The flight attendants say that on July 14, United Airlines ignored a "serious and credible threat to the security and safety of passengers by ordering them to fly in order to avoid cost and disruption to the airline's flight schedule and revenue," said David Marshall, the attorney representing the group.

Also in the federal complaint, the flight attendants allege they informed the airline they were "uncomfortable flying unless United took steps to address this security threat, including deplaning 300-plus passengers and conducting a thorough security inspection of the aircraft."

Marshall said they wanted to "ensure that no explosive device had been planted in the plane."

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