Los Angeles:
Southwest Airlines said on Thursday it called in federal investigators to probe mysterious markings found on the fuselage of some of its planes.
The markings are being treated as vandalism, and do not appear linked to "any known group or activity," it said in a statement, after reports that the markings appeared to be Arabic or Arabic-like.
"Beginning in February, we began to receive reports of vandalism on our aircraft. These markings appear on the exterior of the aircraft and vary in appearance," the airline said.
"Upon discovery, we immediately contacted federal law enforcement and intelligence agencies, which all concluded that these markings have no affiliation to any known group or activity," it added.
A spokesman declined to go into any further detail, but Arizona-based television channel ABC15 cited multiple sources as saying that the markings appear to be Arabic words.
The airline however appeared sceptical about that, saying: "These markings are considered vandalism, and Southwest is conducting an internal investigation to determine who is responsible."
"Southwest takes this behaviour very seriously, and we will continue to involve local and Federal law enforcement agencies as needed until the situation is resolved."
A spokeswoman for the Federal Bureau of Investigation, which media reports said had been called in to help probe the markings, did not immediately respond to a request for more details.
Security across the United States has been tightened in recent weeks due to the 10th anniversary of the September 11, 2001 attacks on New York and Washington.
Southwest added: "We want to reassure customers and employees that safety is always our primary consideration, and the safety of our aircraft is in no way impacted by this act of vandalism."