CHICAGO - United Airlines fired 13 flight attendants who refused to fly on a plane after "someone had written threatening words and drawn menacing images" on the aircraft near its tail, according to a complaint filed this week with the US Occupational Safety and Health Administration.

The words "BYE BYE" were written in six inch letters in oil residue on the jumbo jet's tail cone above two faces, one smiling and another "with a more troubling expression that could be described as frowning or devilish," according to the complaint.

'Bye Bye': The words scrawled on the side of the jet that prompted airlines staff to refuse to fly.

The 13 flight attendants from the July 14 San Francisco flight on a 747-400 aircraft bound for Hong Kong refused to fly, fearing a security threat. They claim they were fired improperly.

United spokeswoman Christen David said the Chicago-based airline will defend the litigation "vigorously."