Taunton Rugby Club, based near the M5 in Somerset, hosted the fireworks on the Friday evening of the crash, which triggered a huge fireball.
Members of the club are said to be in full co-operation with police.
The motorway re-opened in both directions for the first time last night following the 34-vehicle pile-up on the northbound carriageway between junctions 24 and 25, near Taunton, at about 20:30 GMT on Friday.
Meanwhile, a father and a daughter and a retired couple were yesterday named as victims of the M5 crash.
Two of the victims were named as wheelchair-user Michael Barton and daughter Maggie. Maggie’s sister Emma and her partner Christopher Burbull were injured on their way home from a funeral.
A young woman who answered the door at the Barton family home in Berkshire wept as she told of her loss. “We are just devastated. I grew up with them and have known them all my life,” she said.
On a social networking site, Donna Gibbs wrote: “Life is too short! I’m in shock!!! RIP Maggie and Mike! Emma get better babe! Never regret a moment and do what makes you happy!! Xx.”
Hundreds of people have also paid tribute to the dead and injured and their families on Facebook.
Retired couple Tony and Pam Adams, who were in their 70s and from south Wales, were killed while travelling back from visiting their daughter in Taunton.
Mr Adams, a 73-year-old retired hospital administrator from Newport in Wales and his wife, 70, had five children and several grandchildren.
Doreen Martin, a close friend and neighbour, said: “It is so tragic – they were such lovely and caring people, devoted to each other and their family.”
The Reverend Andrew Willie said he had to tell St Mark’s Church in Newport, which the couple attended, that they had died and the congregation was “shocked and deeply saddened”.
“We will remember Tony and Pam with love and affection,” he said.
“Prayers were said for them and their families who are obviously devastated by the loss of their adored parents.”
Three lorry drivers travelling from Cornwall were all understood to have been killed or seriously injured.
Avon and Somerset Police Assistant Chief Constable Anthony Bangham, said his focus was the Friday night fireworks event, and people who had attended the display 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 added: “Accountability is clearly something we will look at when you’ve had an incident such as this.”
He also said “despite our worst fears” the death toll had not risen above seven but some victims may have been so badly burnt that almost no trace of their bodies remained.
Constable 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.
The investigation would look at how the fireworks event was organised, what permission was given for it and what was and was not meant to be happening.
Police said a 40m (131ft) stretch of the carriageway had been damaged by fuel spillage and a 60m (197ft) stretch by “intense fire”. All the vehicles involved in the crash have now been removed from the scene.
Transport Secretary Justine Greening said the police still have a “harrowing” job
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.
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 1000 people observed a minute’s silence at the carnival statue at 12:00 GMT on Sunday.