It is feared that at least seven people died and at least 51 were injured in a massive pile-up on the M5 in southwest England.
The crash, which happened at about 8.30pm yesterday, led to a "massive fireball" at the scene near Taunton in Somerset.
The emergency services are still trying to establish exactly how the crash happened.
Eyewitnesses say there was fog and heavy rain in the area at the time.
Another theory being considered is that drivers may have been distracted by fireworks at the nearby Taunton rugby club.
A spokesman for Devon and Somerset Fire Service described the crash as "the worst road traffic collision anyone can remember".
The fire service said 20 "light vehicles" were involved in the incident, along with six articulated trucks.
Assistant Chief Constable Anthony Bangham from Avon and Somerset Police said a number of passengers had been unable to escape from their burning cars.
"Tragically, a number have lost their lives," he said.
"The emergency services have been working tirelessly. The incident was very, very challenging and 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."
A "comprehensive and thorough" investigation into what caused the tragedy will be carried out, he added.
A huge force of police, fire and ambulance crews was called to the scene and emergency workers are still working to recover bodies and clear the debris.
Firefighters who scrambled around 15 appliances to the scene battled to rescue motorists by cutting people from cars and trucks using hydraulic equipment.
Television footage showed motorists trying to pry open vehicle doors in an attempt to rescue those trapped.
The force of the collision sent debris into the other side of the carriageway causing 20km tailbacks.
The motorway is likely to remain closed in both directions for some time, police said.
