Federal agencies stepped in on Thursday to help Southwest Airlines determine who has vandalized their aircraft with mysterious Arabic markings.

The underbellies of several Southwest 737 passenger aircraft have been vandalized with mysterious writings since February, according to CBS Los Angeles station KNX 1070.

The writing of Arabic or Arabic-type symbols was done with what appears to be some sort of chemical process that reveals the text once an auxiliary power unit is turned on and heats up the outside skin of the aircraft, KNX 1070 reports.

The markings are most commonly found on the engine and landing gear.

Southwest has been aware of the problem for some time now, but reportedly ordered its employees not to discuss the matter with the media.

A company-wide internal memo from Southwest Airlines Executive Vice President Mike Van de Ven, obtained by ABC 15 in Phoenix, explains that these unauthorized markings typically appear as symbols or words tagged on the exterior of the aircraft.

We need our employees to be aware of a situation that is taking place. Beginning in February, we received reports of markings on our aircraft, and recently these reports have increased, it said.

The letter also asks the staff to be vigilant about any suspicious activity.

A source told ABC 15, who first broke the story, that the markings are believed to be the work of an employee or group of employees joking around.

One employee told ABC15 it's not uncommon for ground crews to write their names or airport in the dirt on the planes, but said the recent markings were different.

Several aircraft are involved and the trend has increased in recent weeks.

Though Southwest has been adamant that the markings are an internal vandalism issue that will not affect passengers or air travel, both the Transportation Security Administration and the Federal Bureau of Investigations are working with Southwest to find the culprit.