A tanker truck carrying a highly flammable gas exploded after crashing into a truck on a crowded motorway in the northern Italian city of Bologna, killing one person.
Police said the accident and subsequent blast injured up to 70 people, some with severe burns.
Part of the raised motorway collapsed in the fireball, closing down a key route between northern Italy and the Adriatic Coast and other points south, including Florence and the capital, Rome.
Authorities revised the death toll down from two on Tuesday.
Footage of the crash showed the tanker slamming into the back of a truck which had stopped in traffic.
The tanker immediately caught fire and another truck appeared to hit the tanker from behind.
After an unspecified amount of time, during which the motorway was cleared of most other vehicles, the truck exploded, causing part of the bridge to collapse and sending a fireball across eight lanes of the motorway and beyond.
The flames were followed by a thick cloud of black smoke and aerial photos showed a gaping hole left in the raised motorway.
The fire also spread to a car dealership beneath the bridge and several other vehicles burst into blames.
“The shockwave from the blast was extremely violent. Many of the injured received cuts from flying glass, which had shattered,” said Giovanni Carella, the emergency coordinator for the local fire service.
Italian prosecutors said the tanker was filled with liquid petroleum gas, otherwise known as propane, which is used as a fuel in heaters, cooking equipment and vehicles.
Firefighters said the extreme heat of the flames caused the raised roadway to collapse, sending a heat explosion which incinerated dozens of cars at a Peugeot dealership nearby.
A woman living about a kilometre from the blast site said explosion caused panic because no one knew what it was.
“I was in the house and I heard a huge bang and screams from the window. I saw a huge plume of smoke,” Crisina Felicani told Sky TG24. “There was panic in the neighbourhood, because it wasn’t clear what happened.”
Many people had evacuated the area, due to the thick black smoke, but also while firefighters checked to see if buildings where windows had been blown out were safe to enter.
“It almost would have been an ordinary accident except for the highly flammable material this truck was transporting,” one firefighter, Carlo Cardinali, told Sky.
Authorities said they were working to finish the on-sight investigation so officials could reclaim the damaged road and adjacent lanes and begin to rebuild the motorway, which has long been subject to major traffic backups.
The accident closed down a key section of a major north-south highway that is heavily used year-round, but especially as Italy heads into next week’s peak summer holiday travel period.
Italy’s infrastructure and transport minister, Danilo Toninelli, said authorities were working to ensure traffic would be able to move smoothly “on such a critical stretch.”
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