Flight attendants who were fired after refusing to fly last summer have filed a whistleblower complaint with the U.S. Department of Labor.

Thirteen flight attendants aboard the July 14 flight from San Francisco to Hong Kong walked off the plane and refused to fly after seeing the words "BYE BYE" and a pair of menacing faces scrawled on the plane's tail cone with oil slick residue.

In the complaint, the flight attendants claim they wanted additional security measures taken, such as a security sweep of the 300-plus passengers aboard the flight.

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.

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

The Associated Press contributed to this report.