Police launched a criminal investigation last night and are looking into whether one of the worst ever British motorway crashes was caused by a bank of thick smoke which drifted across the M5.

The 34-vehicle pile-up, which triggered a giant fireball on the M5 in Somerset, claimed seven lives and left 51 injured on Friday evening.

Among the dead were wheelchair-bound father Michael Barton and his daughter Maggie. Her sister Emma was believed to have been left in a coma. The group had attended a funeral just hours before being caught up in the crash.

Retired couple Tony and Pamela Adams, aged 73 and 70, from Newport in South Wales, were also killed in the crash. The devoted churchgoers were on their way home from visiting their daughter Tonia in Taunton. Officers are now focusing their inquiry on a nearby fireworks display which took place around the time of the fatal accident.

Avon and Somerset Police said witnesses had reported “significant smoke” across the carriageway that was “impossible to drive through.”

The probe into what caused the tragedy will be a crime investigation as well as a road policing one, with major crime teams looking into what happened, it was announced.

Assistant Chief Constable Anthony 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.”

He said while driving conditions were “difficult” and there was fog in the area, witnesses had described “very significant smoke across the carriageway” which was “very distracting and very difficult to drive through”.

Mr Bangham added: “We will be doing everything we can to find out as quickly as we’re able to what’s behind that.”

Witnesses at the fireworks display – held at Taunton Rugby Club – 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 caused them to brake.”

The rugby club, which has now been cordoned off by police, has said previously that its display was over by 8.15pm.

The motorway remained closed in both directions between junctions 24 and 25 throughout the weekend. The accident took place at about 8.25pm on the northbound carriageway.

Neighbours of Michael Barton and his family were said to be in deep shock as news of the tragedy filtered back to the Berkshire street where they had lived for nearly ten years.

One woman, living next door, said that Michael, believed to be aged in his late 40s or early 50s, had been confined to a wheelchair and had been “quite poorly” for sometime.

Three lorry drivers travelling from Cornwall are among those thought to have been killed or seriously injured in the carnage, described by one witness as being “like Afghanistan”.

Photographs of the scene clearly showed the charred remains of a lorry from Ginsters, from Callington in South East Cornwall, amidst the carnage.

Another lorry from Samworth Brothers, which owns the Cornish pasty brand, was also gutted by fierce blazes following a number of explosions on the carriageway.

Three drivers, who were travelling from the company’s depot in Callington when the incident happened on Friday night, are thought to be among those killed or seriously injured.

Former employee Chris Tomlinson tweeted: “God bless my three former colleagues who were injured and killed on Friday in the M5 crash.”

A spokesman for Samworth Brothers haulage added: “We are aware that two Samworth Brothers lorries were involved in the crash. One of these was a Ginsters lorry and one was a Samworth Brothers Distribution lorry. For all other enquiries we have been asked to direct people to the police at this time.”

The site of the fireworks display at the rugby club was cordoned off yesterday, with police tape placed around several of the pitches.

Nine police vehicles were parked at the club, suggesting a large forensic team was already in place, and a police photographer could be seen taking pictures at a number of points around the site.

All vehicles involved in the crash have been removed.