LOS ANGELES (CBS) — Federal agencies were working with Southwest Airlines on Thursday to determine who has been vandalizing their aircraft with mysterious markings.

CBS radio invstigative reporter Charles Feldman has learned that since February, several Southwest jets have been vandalized with mysterious writings that show up on the underbelly of their 737 passenger aircraft.

The writings — which some have interpreted as being either Arabic or Arabic-type symbols — appeared to have been done with a chemical process that reveals the text once an auxiliary power unit is turned on and heats up the outside skin of the aircraft, according to Feldman.

While it remains unclear how many aircraft are involved, the trend has reportedly increased in recent weeks.

Southwest Airlines has ordered its employees not to discuss the matter with the media, Feldman said.

In a statement to CBS, Southwest said it is working with authorities to find those responsible for what the company called “vandalism”, but it’s not clear how many people are involved.

Both the Transportation Security Administration and Federal Bureau of Investigations is working with Southwest, but the airline denied this is a safety issue and would not provide any more details.