MILAN  — A tanker truck carrying a highly flammable gas exploded Monday after rear-ending a stopped truck on a crowded highway near the northern Italian city of Bologna. At least two people were killed, up to 70 injured and part of the raised expressway collapsed in the fireball.

Police said the accident and subsequent fire and explosion injured between 60 and 70 people, some with severe burns. It also shut down a highway north of the city that is a key route between northern Italy and the Adriatic Coast and other points south, including Florence and Rome, the capital.

A police video showed the tanker failing to slow down quick enough and plowing into the rear of a truck stopped in traffic, with flames exploding on impact. Another truck appeared to hit the tanker from behind. After an unspecified time lapse, during which the highway was cleared of most other vehicles, the truck erupted in a second, enormous explosion that spanned eight lanes of the highway and beyond.

Flames shot up into the air, followed by a thick black cloud of smoke. Aerial photos showed a gaping hole left in the raised highway. TV images hours after the flames were doused showed the burned-out skeletons of hulking tractor trailers.

Italian prosecutors said the tanker was filled with liquefied petroleum gas, otherwise known as propane, 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 that incinerated dozens of automobiles on new car lots nearby.

Carabinieri said the force of the explosion blew out windows on their barracks, injuring some officers with flying glass.

A woman living about a kilometer (half a mile) 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 neighborhood, 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,” firefighter Carlo Cardinali told Sky.

Authorities said they were working to finish the on-sight investigation so highway officials could reclaim the damaged road and adjacent lanes and begin to rebuild the roadway, 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 that traffic would be able to move smoothly “on such a critical stretch.”

In a separate vehicle accident in the southern Italian region of Puglia, 12 farm workers were killed Monday when the van they were traveling in collided with a truck and overturned, ANSA reported.

The accident was similar to one on Saturday, when a truck laden with tomatoes crashed into a white van and four migrants died.

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