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 reduced visibility and 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 as their "main line of inquiry".
They will now be looking at how the event was organised and who gave permission for it to go ahead.
Meanwhile the motorway has partially re-opened southbound ahead of the police concluding their search of the scene.
Officials said that the entire stretch is expected to be re-opened overnight after a final police sweep and road repairs on on Sunday or Monday morning.
Assistant Chief Constable Anthony Bangham said in a press conference on Sunday that officers now believe it was smoke that contributed to the cause of the crash, and not just fog as previously suggested.
The investigation will now also incorporate a criminal inquiry, Bangham said, adding that "accountability is clearly something we will look at".
Bangham said: "What I am now able to say this afternoon is that our main line of inquiry has now moved towards the event that was on the side of the carriageway and we do believe that while there was fog and it was difficult conditions in the area, that actually from witness evidence there was very significant smoke across the carriageway that in effect caused a bank similar to a fog bank, which was very distracting and very difficult to drive through.
"We will be doing everything we can to find out as quickly as we're able to what's behind that."
"The vehicles and people who were entering into the smoke bank have just described it as being impossible to drive through and that of course causing them to brake. So we know there was brakes and then there was the impacts."
The rugby club has said previously that its display was over by 8.15pm. The motorway remained closed in both directions between junctions 24 and 25 on Sunday following the incident, which took place at about 8.25pm in wet and foggy conditions on the northbound carriageway.
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."
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.