PHOENIX - Southwest Airlines management is asking its employees to be on the lookout for people marking or tagging its planes while parked at the gate.
According to an internal memo obtained by ABC15, "These unauthorized markings typically appear as symbols or words tagged on the exterior of the aircraft."
On Monday afternoon, a Southwest Airlines spokesperson confirmed to ABC15 federal investigators are involved but wouldn't elaborate on what exactly is being written on the 737s.
Multiple sources, who asked not to be identified because they're not authorized to speak on behalf of the company, told ABC15 the markings appear to be Arabic words.
The sources believe the markings are being done by 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.
The markings have shown up on the underbelly of dozens of planes and according to the memo, "commonly on the engine and/or landing gear."
In the memo, titled "A Message from Mike Van de Ven: Your Vigilance is needed," the four paragraph email calls on employees to notify the corporate security team if they witness or have any information on the markings.
"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.
A Southwest Airlines spokesperson made it clear there is no threat to passengers and no impact to travel, calling it an internal situation.
While sources say the FBI is involved, the airline would not confirm which government agency is assisting in locating the person writing the symbols on the planes.
It appears as the markings are written in thick dirt and soot that gathers on the underbelly of the plane and can easily be washed off.
The memo calls it an "internal vandalism issue", stating the marking or tagging of aircraft will not be tolerated.
"This issue is taken very seriously, and we will continue to work in cooperation with both local and federal law enforcement agencies until it is resolved. Any assistance you may be able to provide in determining who is responsible is appreciated," the memo said.