A repair project was launched in 2016 but works only began in September
A repair project was launched in 2016 but works only began in September AFP

The Venice overpass from which a bus plunged this week, killing 21 people, had been earmarked for renovation for years and had a hole in the barrier which was due to be closed, it emerged Thursday.

A repair project was launched in 2016 but works only began in September and had yet to reach the section of the bridge where Tuesday evening's tragedy occurred, the city's transport councillor said in various newspaper interviews.

The electric city bus, carrying tourists back from Venice's historic centre, is believed to have scraped along the edge of the barrier on the side of the road for several metres before reaching a 1.5-metre (four foot) gap.

A second metal fence on the edge of the bridge provided little resistance, and the bus fell 10 metres to the ground, landing upside down next to train tracks, crushing the passengers inside.

An investigation is underway into what exactly happened, with speculation focusing on a possible illness of the driver, an Italian national who died in the crash.

But Italy's national newspapers Thursday highlighted the potentially deadly role of the gap in the road barrier, with La Stampa calling it a "scandal".

Venice councillor Renato Boraso said the gap was put into the barrier when it was built half a century ago in order to give maintenance crews access.

"The barriers are not compliant with current laws," he acknowledged, adding that the gaps were due to have been closed next year as part of the repair works.

The crash has reignited debate over Italy's crumbling infrastructure, but Boraso denied the Venice authority was to blame.

He said that since 2016, renovating the overpass had been a priority but the process of contracting out the seven-million-euro project was "long and complex".

"Let us ask ourselves why such a procedure for works has to be so long in Italy," he added.

Venice authorities declared three days of mourning for the tragedy, which killed 20 foreign tourists in addition to the driver.

Among them were nine Ukrainians, four Romanians, three Germans, two Portuguese, one South African and a Croatian, according to the Venice mayor's office.

Fifteen people remain in hospital, including six Ukrainians, and of those, 10 are in intensive care, including a child, the region said.

The crash occurred at around 7:30 pm on Tuesday evening, on an overpass straddling a railway line and linking the mainland Mestre and Marghera districts.

Locals lined up Thursday at Mestre town hall to sign a book of condolences, with one writing that there were "no words to describe something that should never have happened".

"It could have been me there, with my tourists," said Pia Karttunen, a 56-year-old originally from Finland, who previously worked as a tour guide.

Several passers-by had rushed to help when the bus crashed, including Bujar Bacaj, 43, a Kosovo national who has been living in Italy for 25 years.

He was in his restaurant nearby when he heard the noise, and thought the bridge itself had come down.

He described to AFP seeing "dust, a lot of dust -- I heard people crying, screaming, calling for help in their different languages".

"I saw a little girl whose head was through a window... I managed to get the girl and a boy and take them away from the site."

The two children were Ukrainian, a brother and sister, "full of blood and asking me for their mother", he said, still emotional two days later.

He said people around him could have done more to help, with many taking photos and videos on their telephones.

Bacaj said he went to a nearby hospital to look for the two children, but couldn't find them.

"I learned yesterday that their mother had died," he said, although her partner survived.