Police probing the M5 crash which killed seven people have said a firework display next to the road is the "major line of inquiry".
Avon and Somerset Police Assistant Chief Constable Anthony Bangham said his focus was the Friday night event.
He said "a bank of smoke" had been across the M5 at the time of the crash.
The northbound carriageway and two lanes opposite reopened on Sunday evening, leaving only the southbound outside lane still closed.
The 34-vehicle pile-up happened at junction 25, near Taunton, at about 20:30 GMT on Friday.
Some 51 people were injured in the crash, which happened on the northbound carriageway. The motorway closed between junctions 24 and 25 after the crash.
A retired couple from south Wales were among the seven people killed, a vicar from Newport said on Sunday.
Tony and Pam Adams, who are believed to have been in their 70s, had been visiting their daughter in Taunton and were driving back to their home in Wales when the accident happened.
Mr Bangham said people who attended the fireworks display, which was staged by Taunton Rugby Club, would be interviewed by officers.
"Our main line of inquiry has now moved towards the event that was on the side of the carriageway," he said.
"We do believe that whilst there was fog and 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."
He said: "Accountability is clearly something we will look at when you've had an incident such as this."
Mr Bangham said the major crime teams and collision investigators from the road policing unit were working on the investigation.
"If there's going to be a joint investigation then it is a crime investigation as well as a road policing investigation," he said.
Mr Bangham said the 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.
Taunton Rugby Club said it was working closely with the police to assist with their investigations.
In a statement on its website it offered "thoughts and prayers to the families and friends of those who have been affected by the tragic incident".
All the vehicles involved in the crash have now been removed from the scene.
Police said a 40m (131ft) stretch of the carriageway had been damaged by fuel spillage and a 60m (197ft) stretch by "intense fire".
Of the injured, 25 were taken to Yeovil District Hospital, 17 of the more seriously injured are being treated at Musgrove Park Hospital in Taunton and nine were treated at the scene.
Twenty-three of those taken to Yeovil have since been discharged.
Musgrove Park Hospital said four people had been discharged and 12 were still being treated.
Police have not confirmed the age or sex of the people killed but Mr Bangham said four of the casualties had been in one vehicle.
Hundreds of people have paid tribute to the dead and injured and their families on Facebook.
Prayers for the victims, their families and the emergency services and medical staff were also said during mass at Taunton Catholic Church.
In the nearby town of Bridgwater about 1,000 people observed a minute's silence at the carnival statue at 12:00 GMT on Sunday.