The Road Infrastructure Agency is taking asphalt samples from the Trakia motorway, where the pile-ups happened, to check the state of the road surface, said engineer Ventsislav Angelov, a board member of the agency.
Several people died in the pile-ups and dozens were injured.
The previous development: 12 dead in Trakia motorway crashes near Yambol, 51 in hospital
"The only thing I can see at the moment is a difference in the shade of the asphalt itself," Angelov said. "There are also deformation elements," he added.
Besides the road samples, the agency will also analyse so-called cores taken from the road. Angelov said all the details will become clear when the results come in.
Two years ago the agency did preventive repairs on this stretch and resurfaced the active lane. According to the regional road office, the agency's records list the section as in good condition.
The agency will also check its records to see when the road markings there were last laid. Angelov said markings on national roads are laid in stages, and stretches without markings are refreshed as a precaution.
He said repairs on a bridge near the crash site affected traffic on the motorway.