‘Explaining why the bridge collapsed is a key task. The central question is whether the Morandi bridge’s original design was more to blame than the failure to maintain or close it.’

Rescue workers have used jackhammers and cranes to lift huge slabs of concrete from the collapsed Morandi bridge in Genoa, as anger mounted over the disaster and some families threatened to boycott a state funeral for victims.

A fire apparently caused by a spark from metal-cutting equipment delayed rescue operations for some hours on Friday, but more than 300 emergency workers, including firefighters using sniffer dogs and heavy machinery, soon resumed their search for the 20 people Genoa’s chief public prosecutor said are still unaccounted for.

“We are trying to find points where we can penetrate these incredibly heavy slabs. Then the earth-moving equipment moves in to create a passageway where the dogs can enter,” one rescuer, Stefano Zanut, told Reuters.

A view of the collapsed Morandi highway bridge in Genoa, Italy.

The hope was that the large chunks of debris may have created a “triangle of survival” where someone could still be alive, he said.

But observers at the scene said the chances of finding more survivors were looking increasingly slim: cars and trucks on the 200-metre stretch of the decades-old viaduct on the road linking the Italian port city to the French border plunged 50 metres when it collapsed in a storm on Tuesday.

Deputy prime minister Matteo Salvini, leader of the far-right League, denied reports the ruling coalition – which has blamed the disaster on Autostrade, which operates and maintains nearly half of Italy’s motorways – was split on the issue, saying it was determined to revoke the company’s toll-road concession.

“There is no division. The cabinet approved launching the procedure to withdraw the concession,” Salvini said, after some government members reportedly indicated they favoured fining Autostrade. But the process would take “some weeks or months”, he said, and would involve hearing Autostrade’s counter-arguments.

Salvini has also hit out at austerity measures imposed by the European Union, claiming they constrained government spending. The European commission said it had given Rome billions of euros to invest in infrastructure maintenance and repairs.

Meanwhile, public anger at what many see as decades of government negligence and underspending on the country’s crumbling infrastructure has been climbing.

As relatives hugged each other at the town hall on Friday and prayed over lines of coffins, La Stampa reported some grieving families would not attend the state funeral and mass on Saturday, which has been declared a day of national mourning.

The newspaper said the families of 17 victims have refused to take part and a further seven have not yet decided whether they will attend. Some are planning private ceremonies instead of the official service at Genoa’s exhibition and trade centre, which will be led by the city’s archbishop, cardinal Angelo Bagnasco, and attended by President Sergio Mattarella and the prime minister, Giuseppe Conte.

“It is the state who has provoked this, by not showing their faces: the parade of politicians was shameful,” said Nunzia, the mother of one of four young Italians from Naples who died as they set off on holiday.

The father of another victim said his son “will not become a number in the catalogue of deaths caused by Italian failures. We do not want a farce of a funeral but a ceremony at home.”

An engineering study commissioned by Autostrade last year warned about the state of the bridge’s concrete-encased cable stays, Italian newspapers reported.

The transport ministry has given the company 15 days to show it met all its contractual obligations, and wants the company to rebuild the bridge at its own expense.

Autostrade has denied scrimping on maintenance, saying it has invested over €1bn a year since 2012. It said it monitored the bridge every quarter as required by law, as well as carrying out extra checks using external experts.

The homes of some of the 600 people evacuated from apartments below the remaining spans of the bridge will be demolished for fear of further collapse, the government announced, adding that they would be found alternative housing although it may take months to re-house everyone.

Relatives pay their respects near the coffin of a victim of the Morandi bridge’s collapse, in Genoa.

One survivor told the Associated Press he was in his car as it plunged amid falling sections of highway and concrete. “It came down, everything, the world came down,” said 33-year-old Davide Capello, who walked away traumatised but physically unharmed.

Capello said he heard “a noise, a dull noise. I saw the columns of the highway in front of me come down. A car in front of me disappeared into the darkness.” He said he saw only grey as dust covered the windows.

After coming to a halt, he used the touchscreen phone in the car to call for help, then climbed out to be greeted by “an unreal silence”. He said he was not sure who else might have survived.

The dead include children, one as young as eight, three Chileans and four French nationals. Fifteen people were injured, five seriously. The French nationals, all in their 20s, had travelled to Italy to go to a music festival, and other victims included a family setting off on holiday and a couple returning from their California honeymoon.

The Morandi viaduct dates from the 1960s and has been riddled with structural problems for decades, leading to expensive maintenance and severe criticism from engineering experts.