Flashing signs near Avellino, outside Naples, had warned of slowed traffic along the stretch of the A116 autostrada, a major highway crossing southern Italy, before the crash occurred yesterday night, said highway police and officials, speaking on state radio.
They said the bus driver, for reasons not yet determined, appeared to have lost control of his vehicle.
Hours after the crash, firefighters said that so far they had pulled out 36 bodies and 11 injured people from the mangled wreckage of the bus, which lie on side in a clearance in a wooded area near a road after smashing through a guardrail and plunging some 30 meters (100 feet) below, the Italian news agency ANSA said.
The bus looked as if it had partially split open.
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Motorists and their passengers whose vehicles were hit by the bus stood on the highway near their vehicles. One car's rear was completely crumpled, while another was smashed on its side. It was not immediately known if there were injuries in any of those cars.
Italian news reports said the bus could hold a little more than 50 passengers and that it was almost filled on the ride back after an excursion from the southeastern Puglia area, which is popular with Catholic faithful who admire Padre Pio, a late mystic monk who was based there.
The bus dove off the highway near the town of Monteforte Irpino in Irpinia, a largely agricultural area about 60 kilometers (40 miles) inland from Naples and about 250 kilometers (160 miles) south of Rome.
A reporter for Naples daily Il Mattino, Giuseppe Crimaldi, told Sky TG24 TV from the scene that some witnesses told him the bus had been going at a "normal" speed on the downhill stretch of the highway when it suddenly veered and started hitting cars. He said some witnesses thought they heard a noise as if the bus had blown a tire.