A group of 13 flight attendants say they were illegally fired by United Airlines after refusing to fly on a Hong Kong-bound plane last July that had a “threatening” message scrawled under its tail engine, according to a complaint filed with the U.S. Department of Labor last week.

In the complaint, filed last Tuesday, they said the words “Bye Bye” and two faces, one smiling and the other “devilish,” were found finger-drawn in oil grime under the auxiliary engine of the Boeing 747-400 plane at San Francisco International Airport.

The flight attendants, all with 18 or more years of experience, said the airline refused to deplane the passengers and conduct a security inspection. They said they disobeyed orders to work, believing the lives of more than 300 passengers and crew on the jumbo jet could be endangered.

After a delay, the July 14 flight was eventually canceled. United accused the flight attendants of insubordination and fired them all, according to the complaint.

Chicago-based United, a unit of United Continental Holdings Inc, said it would fight the pending litigation vigorously. A spokesman told Reuters in an email that the airline’s teams had investigated the issue and found there was no “credible security threat.”

The airline followed all of the Federal Aviation Administration’s procedures, the spokesman said, noting that the pilots and mechanics had deemed the plane safe to fly.

The attendants launched their complaint under a federal whistleblower law that prohibits retaliation for reporting possible violations of aviation safety or security.

They are asking to be reinstated and seeking back pay and other damages.

“These flight attendants refused to let an airline bully them into flying because of what they believed, correctly, was a security threat,” their attorney, David Marshall, said in an interview.

The workers said in the complaint that they had been on heightened alert given that, one week earlier, federal authorities announced enhanced security screening due to the threat of onboard explosive devices.

They also said that the images and words might have been drawn on the fuselage at the plane’s previous port-of-call, in Seoul, South Korea, meaning the airline would not have conducted a thorough security sweep prior to takeoff there.

(Reporting by Andrew Chung; Editing by Ted Botha and Leslie Adler)