A group of flight attendants say United fired them for refusing to fly on a Hong Kong-bound 747 that had the words "BYE BYE" and a frowning face mysteriously written in oil on the fuselage.

The 13 former employees sued the airline over the July 14, 2014 flight, which they say was preparing for takeoff at the San Francisco airport when workers discovered the six-inch-tall graffiti written on the tail about 30 feet off the ground.

Via CBS:

In the wake of the disappearance of Malaysia Airlines Flight 370 four months earlier, Lam and the crew said they perceived the message as a serious threat.

But according to the 26-page complaint, United Airlines refused to deplane Flight 869, and the ground crew inspected only the auxiliary power unit, or APU, near the drawings, and said it was a "joke."

The flight attendants refused to fly without a full security sweep, and the trip that was was canceled since United had no crew.

Two months later, they were all fired for insubordination.

According to Bloomberg, the complaint alleges the graffiti was about 30 feet off the ground on a plane located in a secure area and "should have triggered a more-comprehensive reaction."

A United spokesperson tells the Chicago Tribune the airline plans to defend the lawsuit "vigorously."