The ringleader of the London Bridge terror gang tried to hire a 7.5 tonne lorry hours before the attack, police 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 rampage last Saturday night, killing eight 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, while a copy of the Koran opened at a page “describing martyrdom” was discovered in the east London bolthole where the 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.”
A huge investigation was launched in the wake of the attack, with 18 arrests so far. Thirteen have been released without charge, while five men aged between 27 and 30 remain in custody.
Searches have been carried out at 12 properties, with eight ongoing. Detectives expect further searches and arrests in the coming days.
Police have made a number of international inquiries as they work urgently to establish how the three attackers knew each other, as well as investigating whether they were helped through funding or any other means.
Mr Haydon added: “We are not looking at the moment for a wider network. It looks as if it is pretty much a contained plot involving the three of them, which is supported by the forensic evidence we’ve got back so far.”
Mr Haydon appealed for vehicle hire and haulage firms with suspicions about rental attempts to come forward as security services try to face down the wider threat.
He also asked for information about anyone buying “odd quantities” of petrol or lighter fluid for “no apparent reason”, as well as changes in behaviour and unusual activity at addresses.
:: Anyone with information can contact police on 0800 789 321 or visit gov.uk/ACT