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 maintenance, is a subsidiary of Atlantia, which is controlled by the Benetton family.
But on Thursday shares were up around 4 percent after Prime Minister Giuseppe Conte appeared more conciliatory.
"Nationalization is not the only answer," Conte told journalists. "We will evaluate this solution but cannot currently exclude a new tender."
The August 14th Morandi Bridge disaster on the main motorway linking Italy and France killed 43 and injured dozens.
Photo: Italian Police/AFP