DNA samples might be used to confirm identities of some of the 21 people killed including three children when a bus crashed and burst into flames in Venice.
Prosecutor Bruno Cherchi said not all the victims were carrying personal documents.
The all-electric shuttle bus carrying 39 tourists from the Italian city to a campsite crashed through a flyover barrier, plunging almost 50ft (15m) in Mestre on Tuesday night.
Three children including a baby are dead, according to officials.
CCTV footage shows the vehicle driving past another bus before toppling off the carriageway.
Mayor Luigi Brugnaro said it was “an apocalyptic scene” as the coach landed upside down.
“I won’t forget what I saw for the rest of my life,” he told reporters.
“Seeing all those people crammed inside a bus, down there, is something you can’t describe.”
One rescuer spoke of a “tragedy of young people, if not very young people, except for a few adults”.
Among the dead, driver Alberto Rizzotto, 40, had an unblemished record.
He had just started his shift shuttling tourists from Piazzale Roma, at the edge of Venice’s famed canals, to a four-star campsite in nearby Marghera district on the mainland.
Five Ukrainians and one German were among those killed. The rest of those who perished have yet to be identified.
Around 15 survivors were hurt, five of them seriously.
Authorities said they include tourists from Ukraine, Austria, Spain, Austria, France, Croatia and Germany.
Of the injured were two 16-year-olds and two younger children, including a girl aged three who suffered serious burns, the local governor said.
Two German brothers, aged seven and 13, are being treated for broken bones in nearby Treviso hospital. Their parents were killed in the accident and the boys were being given counselling.
Mr Cherchi said only three or four survivors had so far been able to talk to investigators by Wednesday evening.
Regional governor Luca Zaia said the circumstances suggested the driver may have been suddenly taken ill.
Migrant worker Godstime Erheneden, 30, was in his apartment overlooking the busy road when he heard a crash and rushed outside. He and flatmate Boubakar Toure, 27, from Gambia, were among the first to enter.
“When we went in, we saw the driver right away. He was dead. I carried a woman out on my shoulders, then a man,” Mr Erheneden told the local newspaper il Gazzettino.
“The woman was screaming, ‘my daughter, my daughter,’ and I went back in.
“I saw this girl who must have been 2 years old. I have a son who is a year and 10 months old, and they are the same size.
“I felt like I was holding my son in my arms. It was terrible.
“I don’t know if she survived. I thought she was alive but when the rescuers arrived, they took her away immediately.”
