A coach carrying pilgrims plunged off a motorway flyover in southern Italy, killing at least 38 people in one of the worst such accidents in Europe in recent years.
Local prosecutors on Monday launched an investigation into possible manslaughter over Sunday evening's accident close to the town of Avellino near Naples, according to Italian media reports.
Rescuers were still battling Monday to extract people from the wreckage.
The vehicle, carrying about 50 people, had been travelling at high speed when it crashed on a busy dual carriageway between Naples and Bari in an area Italian media described as an accident black spot.
It rammed several cars before plunging through a crash barrier and down a steep slope before coming to a stop on its side off the road about 50 kilometres from Naples.
The driver was among the dead and at least a dozen people were injured, including children. There were reports that some people in the dozen or so cars caught up in the chaos had also been hurt.
Italian news agency ANSA said the manslaughter probe would look into the possible role of the driver in the accident, as well as the state of the coach and the crash barrier on the highway.
Ansa said the driver's body would be examined for the possible presence of alcohol or drugs while traffic police have seized the vehicle documents from the coach operator Mondotravel.
An AFP photographer at the scene described rescue workers searching the crash site early Monday under arc-lights set up around the wreckage of the coach.
"The situation is critical. Our men are working to save as many lives as possible," fire chief Pellegrino Iandolo told Sky TG24.
Rescue workers said they had pulled 33 bodies from the wreckage and found three more of people thrown from the vehicle as it plunged 30 metres down a slope.
Another two died in hospital of their injuries.
"We are still trying to extract people from the vehicle," a police spokesman said. "Our priority now is to free the wounded."
Photographers at the scene described how fire crews raced to find any remaining survivors, as the victims were laid out under white sheets along the roadside.
They said about a dozen wrecked cars littered the highway.
"Looking down from the overpass, the scene of the tragedy: some 30 bodies covered by white sheets, lined up along the roadside," said Cesare Abbate of Italy's ANSA news agency.
From time to time, rescue workers called for "a moment of silence" to listen for signs of life from the wreckage, he said.
One survivor, quoted by his uncle who met him in hospital, reported hearing a tyre exploding and that the driver had been unable to control the vehicle.
The passengers had been returning to Naples following a pilgrimage to Pietrelcina, the birthplace of Saint Pio, an Italian priest canonised in 2002 who is highly venerated in southern Italy.
The Naples-Bari highway has been closed to traffic, the police said.
The last major coach accident in Europe was in March 2012 in Switzerland, when a coach carrying Belgian schoolchildren home from a skiing holiday crashed, killing 28 people, including 22 children.
The accident also comes just days after a train crash in Spain last Wednesday which killed 79 people, the deadliest rail disaster in the country in decades.
The driver appeared in court on Sunday on 79 counts of reckless homicide over the crash near the pilgrimage city of Santiago de Compostela, northwest Spain.