(Releads with prosecutor's comments)
By Paola Balsomini
GENOA, Italy, Sept 6 (Reuters) - Italian magistrates are investigating 20 people for alleged manslaughter after 43 people died when a motorway bridge collapsed in the port city of Genoa last month, the chief prosecutor said.
A judicial source said separately that they included top executives of Autostrade, the company that runs the motorway, and its sister engineering company Spea.
The two companies - both units of Atlantia, the infrastructure group controlled by Italy's Benetton family - are under investigation as under Italian law firms are also held responsible for their employees' actions.
Genoa Chief Prosecutor Francesco Cozzi told reporters all the suspects had to be notified that they were under investigation before their names could be made public.
A 200-metre section of the Morandi bridge gave way on Aug. 14, plunging dozens of vehicles into the valley below.
The disaster provoked public outrage, and Rome's anti-establishment government piled blame on Autostrade, which operates the A10 motorway linking Genoa to the French border where the bridge was located.
Senior officials at Italy's infrastructure ministry are also being investigated, the source said.
Under Italian rules, the motorway operator is responsible for maintenance, while the ministry has a supervisory role.
Autostrade had no immediate comment. The ministry was not immediately available for comment. Autostrade has said in the past it carried regular checks on the bridge as required by the law.
Built during the post-war boom like much of Italy's infrastructure, the Morandi bridge had been shown to be defective soon after its completion in 1967.
Autostrade had been planning to strengthen the stays of the bridge and had launched a tender in April for the contract after obtaining the necessary approval from the ministry. (Reporting by Paola Balsomini, writing by Valentina Za; editing by David Stamp)