In a document on the probe's findings seen by Reuters, prosecutors said the collapse was triggered by the rupture of the load-bearing cables inside the stay of the bridge's ninth pillar, which were eaten away by a highly corrosive atmosphere over the 51 years of the bridge's life.
Managers at Atlantia units Autostrade per l'Italia and SPEA allegedly avoided proper checks of the state of the infrastructure and did not correct serious issues that started to emerge only a few years after the viaduct opened in 1967, the document showed.
"In 51 years of the bridge's life not the slightest maintenance work was carried out to reinforce the stays of the ninth pillar," the document said.
Between 1982 and the collapse in 2018, structural works on the viaduct cost 24.6 million euros and Autostrade paid for less that 2% of them, it added.
This situation "cannot be justified by the inadequacy of the necessary financial resources at Autostrade," prosecutors said, adding that the company ended in black each year between 1999 and 2005.
The document added that Autostrade's annual net income was at least 586 million euros in the period between 2005 and 2017, when the company distributed 80% or more of its profits to its shareholders.
During their 2-1/2 year probe Genoa prosecutors placed 69 individuals, including former Atlantia Chief Executive Giovanni Castellucci and the former head of Atlantia's engineering unit SPEA Antonino Galata, under investigation for suspected crimes including manslaughter and wilful disaster.
Former and current officials at the transport ministry allegedly failed to oversee necessary maintenance, prosecutors said.
Under Italian law, once an investigation has been closed, suspects have three weeks to respond. At that point, prosecutors can seek court authorisation to press charges or recommend shelving a case.
Atlantia, Autostrade and SPEA declined to comment on the issue. Lawyers representing Castellucci and Galata did not comment on the issue. Italy's transport ministry was not immediately available for a comment.
Under Italian law, firms can be held responsible for their employees' actions.
Atlantia, controlled by the Benetton family, is in talks with a consortium of investors led by Italian state lender CDP to sell its 88% stake in Autostrade under a plan sponsored by the government to regain control of the motorway company.
Castellucci, who was behind Atlantia's international expansion, was ousted from the infrastructure group in late 2019 after 13 years at its helm.
Prosecutors in Genoa have started three other investigations linked to the bridge collapse revolving around allegations of falsified road safety reports, improper installation of anti-noise barriers and poor upkeep of road tunnels.
Autostrade per l'Italia, which counts Chinese fund Silk Road and Allianz among its investors, runs half of Italy's 6,000 km (3,730 miles) of toll roads, generating a significant share of Atlantia's core earnings before the disaster.
Shares in Atlantia were down 0.7% at 1055 GMT, underperforming Milan blue-chip index which was up 0.5%.
(Reporting by Emilio Parodi and Francesca Landini; Editing by Crispian Balmer, Jan Harvey and Edmund Blair) ((email@example.com; Reuters Messaging: firstname.lastname@example.org))