The ringleader of the London Bridge terror gang tried to hire a 7.5 tonne lorry hours before the attack, police have revealed.
Detectives suspect the carnage inflicted could have been even worse if Khuram Butt had not failed to secure the vehicle because his payment did not go through.
Instead he resorted to "plan B" and rented a white van which ploughed into pedestrians as the perpetrators launched their deadly rampage last Saturday night, killing eight victims and injuring dozens more.
After leaving the vehicle, the terrorists used 12-inch ceramic knives with pink blades in a stabbing spree.
It also emerged that multiple petrol bombs were found in the van used in the outrage, while a copy of the Koran opened at a page "describing martyrdom" was discovered in the east London bolthole where the three men plotted the attack.
Commander Dean Haydon, head of the Metropolitan Police's Counter Terrorism Command, revealed that Butt had attempted to hire a lorry online on the morning of the atrocity.
He said: "Because of the fact his payment method failed he couldn't get hold of that lorry.
"My view at the moment is that he then went to plan B and ended up hiring the van instead."
The van travelled over the bridge twice before it was driven at speed at pedestrians.
It is thought three victims were killed on the bridge - including one man who was thrown into the Thames - before the attackers left the vehicle and stabbed five people to death around Borough Market.
Police believe Butt was driving the van.
Mr Haydon said: "When I come back to Butt trying to get hold of a 7.5 tonne lorry - the effect could have been even worse."
The potential for large vehicles to inflict mass casualties was laid bare in horrifying fashion last year when a lorry drove through crowds gathered to celebrate Bastille Day in Nice, killing 86 people and injuring scores more.
Fears that the London Bridge trio planned an even more deadly strike were also heightened by the discovery of items in the white Renault van they used.
In the back of the vehicle officers found 13 wine bottles filled with flammable liquid with rags wrapped around them in the shape of Molotov cocktails, along with two blow torches.
Mr Haydon said: "It's feasible when you look at their actions, they were still fairly close to the van, there is a possibility they could have come back to the van.
"Was there a plan B that, after they had stabbed individuals, were they then planning to come back to the van and ignite the Molotov cocktails and that was a secondary attack?
"We don't know - I can only surmise."
Police also found a number of office chairs, bags of builders' gravel and a suitcase in the van.
Detectives believe the gravel may have been placed in the vehicle to make it heavier, or as part of a cover to justify hiring it, while the chairs may have been used to convince family and friends they were moving furniture.
Butt, a 27-year-old Pakistani-born British citizen, and his two accomplices, Rachid Redouane, 30, who claimed to be Moroccan-Libyan, and Youssef Zaghba, a 22-year-old Italian national of Moroccan descent, were shot dead by armed police eight minutes after the first emergency call.