Witnesses to Friday night's pile-up on the M5 motorway have described a "smoke-bank" created by a nearby fireworks display which made driving conditions "impossible".
The thick cloud of smoke could have caused drivers to swerve and brake, police said, contributing to the major series of crashes that killed seven people and injured another 51.
Avon and Somerset Police are now investigating the fireworks and bonfire event at Taunton Rugby Club, and will be looking at how it was organised and who gave permission for it to go ahead.
Assistant Chief Constable Anthony Bangham said in a press conference on Sunday that the investigation will also incorporate a criminal inquiry, and said that "accountability is clearly something we will look at".
The Transport Secretary Justine Greening has said the government will also investigate the causes of the pile-up, which involved 34 vehicles and in which at least seven people are known to have died.
"Some people have said that the weather was particularly bad, but I think clearly what we need to do at the moment is focus on the NHS work that's happening to take care of the people who have been injured, but also getting the motorway back open again for the public as soon as possible. I think we'll have a debate about the policy issues in the coming weeks."
The motorway remains closed between junctions 25 and 24 northbound and southbound between junctions 23 and 25. Officials said that the stretch is expected to be re-opened overnight after a final police sweep and road repairs on on Sunday or Monday morning.
Road safety charities including the Royal Society for the Protection of Accidents have said the crash shows that the government's plans to increase the speed limit on motorways to 80mph should be abandoned.
However Tory MP Margot James told Jeremy Vine on BBC One there shouldn't be a rush to judgement on the speed limit, saying: "We don't know fast people were driving. I don't think people were driving that fast anyway, from the reports I've heard from the people interviewed who survived the accident. So I'm not sure it was the speed that was causing the problem. The fog was obviously a problem, but we don't know yet."
Emergency workers have described the crash, involving around 27 vehicles, as "the worst road traffic collision anyone can remember."
According to police, a "number" of passengers perished when they were unable to escape from their burning cars.
Assistant Chief Constable Bangham described the devastating crash as "very very challenging" for emergency workers.
"On arrival crews were faced with literally one massive fireball. Most vehicles were well alight and most continued to burn for a considerable time.
"This made it very difficult to search the vehicles. Some of them have been burned to the ground."
Around 15 fire engines were called to the scene and crew battled to save trapped motorists by cutting people from cars and lorries. Television footage shows members of the public trying to open car doors in an attempt to rescue those trapped.
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