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.