ROME—A busy overpass in the northern port town of Genoa collapsed shortly before noon Tuesday, sending more than 30 cars, campers, and semi trucks nearly 300 feet down to the ground below.

The horrific accident happened during a torrential rainstorm on the eve of Italy’s biggest summer holiday, “Ferragosto,” on Aug. 15. Several witnesses reported seeing lightning strike the area just before the accident. Police said violent winds and lightning may have contributed to the collapse.

Dozens of cars screeched to a stop on both sides of the dual carriageway highway at the edge of the breaking points. The bridge, which was built in the 1970s, was being repaired over the last several weeks.

Luca Cari, the head of Italy’s civil protection, said they were treating the search and recovery much like an earthquake as they sort through concrete for signs of life. So far, local media has reported at least two people have been pulled from their cars alive.

The search and rescue efforts have been greatly hampered by the storms, which are expected to continue over the next 48 hours.

Italian officials said at least 20 people have died, but that number is expected to rise. The nationalities of the dead and injured have not been confirmed.

Nic Robertson, CNN’s international diplomatic correspondent, was on holiday in the area when traffic suddenly stopped as his own car approached the bridge. He described a scene of complete mayhem as the lightning storm struck, the bridge collapsed, and people ran from the bridge in sheer panic.

Miraculously, the bridge did not at first appear to have fallen on any of the residential buildings below the bridge, which spans railroad tracks and the Polcevera stream. Several hundred people were evacuated from the area below the bridge.

The road is a major thoroughfare linking the city of Genoa to the airport and joins a coastal highway that leads into France.