Assistant Chief Constable Anthony Bangham said: "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."
Witnesses at the Taunton Rugby Club fireworks display were being interviewed, he said, adding that accountability was "clearly something we will look at".
He went on: "We believe from the witnesses and from what we're being told that it was smoke and not fog.
"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."
The police investigation will look at how the fireworks event was organised, what permission was given for it and what was and was not meant to be happening there, Mr Bangham said.