Thirteen flight attendants filed a federal complaint Wednesday against United Airlines saying they were fired for refusing to fly from San Francisco to Hong Kong on a plane with the words "BYE BYE" and a pair of faces scrawled on it, their attorneys said Wednesday.

The flight attendants filed the whistleblower complaint with the U.S. Department of Labor, saying the drawings included images that could be construed as "menacing" or "devilish."

The drawings were traced in an oil slick on the plane's tail cone. They included one smiling face but another that appeared to show eyes closed and a tongue sticking out.

The complaint says the flight attendants sought additional security measures and wanted the plane's more than 300 passengers taken off the July 14 flight for a sweep.

It says they were fired in retaliation for "exactly what the flying public would expect from a group of highly experienced airline professionals."

The flight attendants said they were working in the wake of the recent disappearance of Malaysian Flight 370 and a possible phone-and-laptop bomb warning a week earlier from the Transportation Security Administration.

"Given the gravity of the risk involved — the lives of passengers and crew alike — 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," flight attendant Grace Lam said in a statement.

United said Wednesday in response to the complaint that the flight attendants' fears were addressed. It said a thorough investigation that complied with Federal Aviation Administration rules was conducted and no threat was found.

"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," spokeswoman Christen David said in a written statement.