NEW YORK (MarketWatch) — Thirteen former United Airlines flight attendants are demanding to be rehired after being fired last summer for refusing to fly over what they said was a safety threat.

A 26-page whistleblower complaint, filed with the Occupational Safety and Health Administration on Tuesday, claims the 13 flight attendants had been scheduled to fly from San Francisco to Hong Kong on July 14, but refused after becoming aware of “threatening words” and “menacing drawings” drawn in the oil-slick coating on one of the plane’s engines.

It described the writing and images as “the words ‘BYE BYE’ in six-inch-high letters above two faces, one smiling and the other with a more troubling expression that could be described as frowning or devilish.”

“We’re asking OSHA to send a strong signal to the airline industry that terminating employees for taking action against security risks is not OK,” said David Marshall, a partner at Katz, Marshall & Banks, a law firm that specializes in whistleblower complaints and labor law, and the lawyer for the flight attendants.

According to the complaint, United, which is owned by United Continental Holdings Inc. UAL, -3.61% , refused the flight attendants’ request to deplane passengers and conduct a security inspection of the aircraft. As a result, the flight attendants refused to fly, the flight was canceled because of a lack of staff and the flight attendants were subsequently fired for what United Airlines saw as insubordination, the complaint said.

United’s flight operations, safety and maintenance teams investigated and determined there was no credible security threat, a company spokeswoman said in an email. All of the safety procedures set by the Federal Aviation Administration and United were followed, including a comprehensive safety sweep prior to boarding, leading the pilots, mechanics and safety leaders to deem the aircraft safe to fly, she said.

United intends to vigorously defend itself in the case, she added.

The complaint was brought under the Wendell H. Ford Aviation Investment and Reform Act for the 21st Century, which protects employees from retaliation for opposing violation of air-safety or air-security standards.

“Given the gravity of the risks 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,” Grace Lam, one of the 13 flight attendants, said in a news release.

The 13 flight attendants are seeking reinstatement, back pay and compensatory damages. OSHA has 60 days to issue a ruling.