Yambol district prosecutors have asked for toll camera footage from the stretch of the Trakia motorway near the town, where two serious pile-ups happened in fog. Twelve people died and 51 were injured. District prosecutor Doychin Doychev told reporters that prosecutors expect the footage to show what the weather was like. They want to know how thick the fog was, how far drivers could see and how many vehicles were on the road when the crashes happened.

The first alert came at about 7:00am on Tuesday 10 November, for the carriageway towards Burgas. Soon afterwards, cars and lorries also collided on the carriageway towards Sofia.

Earlier report: At least 11 dead and 50 injured in crashes involving 86 vehicles on the Trakia motorway

Doychev said nobody can be charged yet because there is not enough evidence. The toll data has not arrived, and investigators will analyse it once they get it.

The Romanian driver of the first lorry in the chain has been detained. Prosecutors want to question him again about why he slowed down, and about the weather and visibility. Doychev said the driver told them there was fog before the crash site and that it suddenly became much thicker. The driver said he slowed sharply to match his speed to what he could see. He did not name an obstacle on the road or another vehicle as the cause.

Witnesses say some cars were travelling fast. Inspections of the scene and damage to some of the vehicles, including those with victims, point the same way, the prosecutor said. "There is simply no visible braking distance, which means the speed was high," Doychev said.

He said this means a speed that did not suit the weather, not necessarily one above the motorway limit. By law, drivers must adjust to the weather and, if needed, slow almost to a stop.

"My personal view is that at first glance this may be human error, a failure to adapt to road conditions and the thick fog," Doychev said. He added that the reasons the crashes were not prevented are complex.

Investigators are also checking whether any institution is responsible for the state of the road and for traffic management. They have asked the Road Infrastructure Agency for documents on the stretch, covering its construction, its opening, traffic management and signs. They will also ask every other body in charge of the carriageway, road surface, markings and traffic for information.

Samples have been taken from the road surface. Investigators will compare the results with findings from construction and technical experts, if such experts are appointed.

Six vehicles have been examined, and forensic medical examinations are under way. More than 50 witnesses have been questioned, and prosecutors expect the total to pass 100.

Prosecutors will order a vehicle technical expert report once they have gathered as many statements and as much other evidence as possible. Doychev said they are delaying it so that experts can give accurate and sound answers on what caused the crash.