The other two pylons were reinforced in the early 1990s, when Autostrade was still owned by the Italian state. Back then Prof Carmelo Gentile was one of those working on the project and he witnessed some disturbing evidence of decay.

"At a certain point they did an inspection of the bridge and a piece of concrete came away and revealed a hole. Inside you could see the steel falling to pieces. In fact, we found an area where there was no concrete at all. If there is an air pocket inside water gets in and corrodes the metal, making it rusty.

"Riccardo Morandi [the bridge's designer] loved the poetry of concrete so he wanted to make a bridge where concrete was all you saw - the metal was encased in concrete. This design contains a huge number of problems. If everything is done perfectly, you protect the steel inside. But if the manufacturing process is not perfect, you can no longer inspect the steel stay, so you cannot see if there is corrosion or if it has become unsafe. And it was very hard with the technology of the time not to have any air pockets or bubbles in the concrete."

Deputy Prosecutor Paolo d'Ovidio says expert analysis of the debris has shown that, in the section of the bridge that collapsed, much of the metal inside the concrete was also badly corroded. His team are investigating possible manufacturing defects as well as the maintenance and safety checks carried out on the bridge.

"The condition of the bridge was very poor. It was a miracle that it hadn't happened the year before. It could have happened three, five, even 10 years before," he says.