Pharr officials are investigating why a tractor-trailer went up in flames Thursday morning. 

The tractor-trailer carrying gravel was traveling from San Juan to Pharr when, for unknown reasons, it became fully engulfed in flames off the expressway near the 1200 block of Gumwood and the Frontage Road. 

The driver escaped from the vehicle safely without any injuries. 

Pharr firefighters quickly put out the flames as Pharr police rerouted traffic in the area.