Thirteen flight attendants who were terminated by the United Airlines said that they were illegally fired after they refused to fly on a plane that supposedly flew from San Francisco International Airport to Hong Kong on July 14 last year.

In a complaint filed with the U.S. Department of Labor on Tuesday, the flight attendants, who have all at least 18 years of work experience, said that they refused to fly because of what they claimed to be a threatening graffiti that was found under the tail engine of flight's Boeing 747-400 plane.

The message, which was written using oil grime, had the words "Bye Bye" along with two faces, one of which was described to have a "a more troubling devilish expression."

The airline workers said that they were on a heightened alert at the time because a week prior to the flight, federal authorities have warned of consumer-electronic-device bombs. The incident also occurred after Malaysian Flight 370 disappeared.

The workers added that the words and images were also likely drawn at the previous port of call of the plane in Seoul, Korea, which means that the airline did not conduct a thorough security check prior to the airplane taking off there.

The group said that they informed the airline about this and about their discomfort at flying unless the company addressed the problem.

The flight attendants likewise pointed out that United Airlines did not deplane the passengers nor conduct security inspections to ensure that no explosive device was onboard, which they believe was required under Federal Aviation Administration (FAA) rules.

They said that they disobeyed the orders to work in the belief that the crews and the 300 passengers aboard the plane could be endangered.

"We were not willing to bow to United's pressure to ignore an unresolved security threat even though the company made clear that we risked losing our jobs," said Grace Lam, one of the flight attendants who were terminated from their job.

The flight was eventually cancelled and their airlines accused the complainants of insubordination, firing all of them.

United spokeswoman Christen David said that the airline conducted investigations and found that there was no credible security threats. David also said that the company complied with all of the required FAA rules citing the pilots and the mechanics finding the airplane safe to fly.

"All of FAA's and United's own safety procedures were followed, including a comprehensive safety sweep prior to boarding, and the pilots, mechanics and safety leaders deemed the aircraft entirely safe to fly," David said.

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