Italian magistrates said Thursday they are investigating 20 people and the company that manages the Genoa motorway bridge that collapsed last month killing 43 people.
They could face charges including manslaughter and failure to comply with safety regulations, magistrates in the north-western city said.
At the end of the probe, prosecutors will decide whether to file charges, including against Autostrade per l'Italia, which manages nearly half of Italy's motorway network.
The company, accused by the government of having pocketed huge profits while investing too little in maintainance, is a subsidiary of Atlantia, which is controlled by the Benetton family.
Atlantia shares have plummeted more than 30 percent since the disaster, with the government threatening to revoke its lucrative contracts or nationalise the network.
But on Thursday shares were up around 4 percent after Prime Minister Giuseppe Conte appeared more conciliatory.
"Nationalisation is not the only answer," Conte told journalists.
"We will evaluate this solution but cannot currently exclude a new tender."
The August 14 Morandi Bridge disaster on the main motorway linking Italy and France killed 43 and injured dozens.