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.

Bologna explosion: truck explodes near airport in Italy Show all 15 1 /15 Bologna explosion: truck explodes near airport in Italy Bologna explosion: truck explodes near airport in Italy An aerial view of the fire caused by a tanker exploding, triggering a series of blasts causing the collapse of a road in Bologna Vigili del Fuoco/EPA Bologna explosion: truck explodes near airport in Italy A view of a partially collapsed section of the motorway bridge of the Casalecchio A1-A14 junction following a road accident in Bologna, northern Italy. EPA Bologna explosion: truck explodes near airport in Italy Burned vehicles at a car park after a major fire spread following the accident Vigili del Fuoco/EPA Bologna explosion: truck explodes near airport in Italy Two persons were killed and at least 60 others were wounded after a tanker exploded triggering a series of blasts causing the collapse of a road, media reported Vigili del Fuoco/AFP/Getty Bologna explosion: truck explodes near airport in Italy epa06931125 A handout photo made available by Vigili del Fuoco, the Italian National Firefighters Corps, shows a view of a partial collapsed section of the motorway bridge of the Casalecchio A1-A14 junction following a road accident in Bologna, northern Italy, 06 August 2018. Two persons were killed and at least 60 others were wounded after a tanker exploded triggering a series of blasts causing the collapse of a road, media reported. EPA/VIGILI DEL FUOCO HANDOUT -- BEST QUALITY AVAILABLE -- HANDOUT EDITORIAL USE ONLY/NO SALES VIGILI DEL FUOCO HANDOUT Vigili del Fuoco/EPA Bologna explosion: truck explodes near airport in Italy Firefighters work at the scene where the tanker truck exploded AFP/Getty Bologna explosion: truck explodes near airport in Italy A firefighter walks past a line of burned vehicles EPA Bologna explosion: truck explodes near airport in Italy The explosion seen at Borgo Panigale Daniele Giovanardi/Reuters Bologna explosion: truck explodes near airport in Italy Emergency services move an injured person EPA Bologna explosion: truck explodes near airport in Italy A fire is seen after an accident caused a large explosion at Borgo Panigale, on the outskirts of Bologna, Italy August 6, 2018, in this picture obtained from social media. Alberto Rodriguez/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. SOCIAL MEDIA REUTERS Bologna explosion: truck explodes near airport in Italy A damaged section of the motorway bridge EPA Bologna explosion: truck explodes near airport in Italy A helicopter transporting water flies over the site AFP/Getty Images Bologna explosion: truck explodes near airport in Italy epa06931071 A view of a partial collapsed section of the motorway bridge of the Casalecchio A1-A14 junction following a road accident in Bologna, northern Italy, 06 August 2018. Two persons were killed and at least 60 others were wounded after a tanker exploded triggering a series of blasts causing the collapse of a road, media reported. EPA/GIORGIO BENVENUTI GIORGIO BENVENUTI EPA Bologna explosion: truck explodes near airport in Italy Firefighters work at the scene where a tanker truck exploded on a motorway just outside Bologna, northern Italy, on August 6, 2018. A tanker truck exploded on a motorway just outside the northern Italian city of Bologna on August 6, engulfing the area with flames and black smoke, the fire service said, with local media reporting one person killed. The explosion occurred near Borgo Paginale to west of the city, very close to Bologna airport, at around 2:00 pm (1200 GMT), the Italian fire service said on Twitter. / AFP PHOTO / --/AFP/Getty Images - AFP/Getty Images Bologna explosion: truck explodes near airport in Italy Injured people receive medical attention AFP/Getty Images

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.