Italian authorities are still trying to identify all of the 21 victims of Tuesday's deadly bus crash in Venice.

The electric bus crashed through a bridge barrier, and plunged almost 15m (50ft) in the mainland borough of Mestre, before bursting into flames.

DNA samples are being used to confirm the identities of those who were not carrying personal documents, prosecutor Bruno Cherchi said.

Three children including a baby were among the dead, according to officials.

The bus was carrying 39 tourists from the centre of Venice to a campsite. On Wednesday evening, relatives began arriving in Italy from other countries to identify the dead.

Mayor Luigi Brugnaro said a huge tragedy had taken place. "An apocalyptic scene, there are no words," he said on social media.

CCTV footage showed the vehicle driving past another bus before toppling off the carriageway.

One rescuer spoke of a "tragedy of young people, if not very young people, except for a few adults".

Nine Ukrainians, three Germans, four Romanians, two Portuguese, a South African national and the Italian driver were among those killed, a spokesperson for Venice's mayor said.

Fifteen people are known to have been injured, five of them seriously - including Ukrainians, Austrians, Spaniards and other foreign tourists, according to officials.

Among the injured were two 16-year-olds and two younger children, the local governor said.

Two German brothers, aged seven and 13, were being treated for broken bones in hospital in nearby Treviso. Their parents were killed in the accident and the boys were being given counselling.

A young Croatian woman who was on her honeymoon also died, Ansa news agency reported. Her husband of about three weeks was sent to hospital.

Some survivors in the Angelo di Mestre hospital were asking for their loved ones, said the head of medicine, Chiara Berti. "There were entire families, grandparents, grandchildren, spouses."

Venice prosecutor Bruno Cherchi said only three or four survivors had so far been able to talk to investigators by Wednesday afternoon.

The city's authorities have declared three days of mourning after the tragedy.

The bus crashed at around 19:45 (17:45 GMT) on Tuesday. It had apparently been rented by a local company to pick up tourists from the historic centre of Venice and take them to a campsite in the nearby Marghera district, on the mainland.

Witnesses saw the bus scraping along the guard-rail on the flyover for 50m, before tumbling to the ground, the prosecutor added.

The bus company emphasised that the 13-tonne vehicle was electric, discounting earlier reports that it also ran on methane gas. Fire brigade commander Mauro Longo told Il Gazzettino website that the bus's batteries caught fire and made the task of clearing the vehicle a complex operation.

Witnesses said they could hear people screaming, but the flames were too intense to get to them.

A 27-year-old Gambian worker and his housemate were among the first people to reach the scene. He told how he had pulled three or four people from the bus, including a young girl.

Boubacar Touré and Odion Eboigbe, who is Nigerian, ran to the scene after hearing a sudden, thundering crash beside their apartment.

"We ran down to the spot where the bus was on fire and I heard a woman screaming, 'My baby, my baby,'" Boubacar told the BBC.

"I managed to pull her through the window and then pulled out her son, who was badly burnt but still alive."

Boubacar said the fire was so intense that fire extinguishers had little impact on the flames.

What is unclear is why the bus left the flyover on a downhill stretch of the road and careered through a guard rail and metal barrier. Police are looking at video from security cameras near the crash site.

The 40-year-old driver, Alberto Rizzotto, had worked for the bus company for seven years and there was no indication on the road that he had tried to brake before the crash.

In his last Facebook post, he said he was running a "shuttle to Venice".

The head of the Veneto region, Luca Zaia, said "everything points" to the driver being taken ill in the moments before losing control of the bus. However, he added that it was prudent not to speculate on the causes of the accident.

Massimo Fiorese, from La Linea bus company, said the vehicle was less than a year old and the driver highly experienced.

"There's a video of the bus just before it falls," he told the Ansa news agency. "The vehicle arrives, slows down and brakes. It's almost at a standstill when it crashes through the guard-rail. I think the driver must have fallen ill, because otherwise I can't explain it."

Firefighters eventually removed the wrecked bus from the scene early on Wednesday.

A reception point staffed by psychologists and psychiatrists has been set up at a nearby hospital to provide support for the victims' families.

Italy's Prime Minister Giorgia Meloni said the country's thoughts were with the victims and their family and friends.