Yes, exactly. The cab? The trailer? They'll try to figure out in the investigation.
A number of delivery companies have been ordering CNG and LNG powered tractors for their over the road rigs. A disconnected fuel line in the engine compartment is a possibility. Some of the mandated diesel particulate systems work with high heat, some of the earlier ones would actually melt the asphalt under idling trucks.
Prayers for the victims and their families.