The precise dynamic which led to a fatal bus crash last night in which 39 people were killed near Avellino, southern Italy remains unclear.
As rescue workers and firemen continue mop-up operations, investigators said too soon to arrive at any conclusions.
Thirty-six people were killed immediately and another three died later in hospital, a police official said.
All of the 10 people who survived the crash are seriously injured and it is possible that the death toll may rise. The dead include five children.
What is known is that a bus containing 48 people, mainly from the area around Naples, failed to slow down when it came upon a traffic hold-up on the Napoli-Canosa autostrada last night.
The bus crashed into 15 cars among the slow-moving traffic in front of it, went out of control and then crashed through a guard rail before falling 30m into the valley below.
The crash came at a point in the motorway where the road goes over a high flyover.
Witnesses amongst other drivers who were stuck in the traffic up ahead of the crash said that the impact of the bus crashing into the ravine was “like an earthquake, a terrible explosion”.
In TV images this morning, the wrecked bus was unrecognisable, having split into two parts as the violent impact of the crash literally tore it apart.
Most of the bodies have been laid out in a school classroom in the nearby village of Monteforte Irpino for identification purposes. Several relatives themselves became unwell as they identified loved ones who had been disfigured in the crash.
Eye witness accounts and the absence of tyre brake marks on the autostrada asphalt suggest that the bus did not slow down as it came upon the traffic hold-up.
The traffic hold-up had reportedly been well signalled both on the ground by motorway personnel and also on the overhead electronic noticeboard system.
Investigators are therefore looking at essentially two possible explanations - namely, mechanical problems such as brake failure or a tyre blow-out or, alternatively, human error on the part of the driver.
The crash happened at about 8.30 yesterday evening at the end of a long and very hot day prompting investigators to wonder if the driver fell asleep at the wheel.
The group of pilgrim had been on an outing which had included visits to the town of Pietrelcina, birthplace of St Padre Pio, and the spa town of Telese Terme.
The Public Prosecutor’s office in Avellino has opened an inquiry into “multiple manslaughter”, whie Italian prime minister Enrico Letta, as a mark of respect, cancelled a scheduled visit this morning to the Acropolis in Athens.
Mr Letta is in Greece for an EU meeting.