Mysterious messages that appeared to be scrawled in Arabic writing on the underbellies of several Southwest Airlines jets were being investigated Wednesday by the airline and the FBI, Los Angeles radio station KNX-1070 reported.
The graffiti, which began appearing in February on 737-model planes, has been found more often in recent weeks, according to the report.
The writing appears to have been etched using a chemical process and is visible only after an auxiliary power unit is turned on.
Southwest Airlines spokeswoman Brandy King confirmed that the company is "conducting an internal investigation." The FBI and Transportation Security Administration were also making enquiries.
The airline denied that the vandalism posed a safety or security threat.