London (CNN) -- Smoke from a bonfire and fireworks show caused a fiery weekend crash on a highway in southwestern England that killed seven people, police said Sunday.
The Friday night pileup involved nearly three dozen cars, and 51 people were injured, said Anthony Bangham, assistant chief constable for the Avon and Somerset police department.
"It was smoke and not fog that caused vehicles and people entering into the area to brake as it was impossible to drive though. So people braked and then crashed," Bangham said.
The heat from burning vehicles damaged a 60-meter (195-foot) stretch of the M5 highway near Taunton, about 240 km (150 miles) west of London, police said. But Bangham said that despite earlier fears, the death toll is not expected to rise.
He said "an event on the side of the carriageway" was the source of the smoke. He did not identify the name of the event or the organizers but said, "We're surely looking at accountability."
"We will look at who gave the permission, what permission had been given and how the event was organized," Bangham said.
The crash occurred on the eve of Guy Fawkes Day, the annual commemoration of a failed plot to blow up Parliament and assassinate King James I in 1605, a holiday usually marked with fireworks and bonfires.
Police were working to clear the highway Sunday evening, and the road is expected to be open in time for Monday morning's rush hour, he said.