Molotov cocktails were found inside a van they used to kill people
The London Bridge terrorists tried to hire a 7.5-tonne lorry online on the morning of their attack, British police have revealed.
But the payment by the gang's leader, Khuram Butt, failed to go through.
Instead, he hired a smaller van and loaded it with bags of gravel, probably to make it heavier so it could be a more effective battering ram.
The van mounted the pavement on the busy bridge last Saturday night, killing three people.
The gang crashed the vehicle into railings and then ran through Borough Market, randomly stabbing more terrified victims at bars and restaurants, before armed officers arrived and shot them dead.
Soon after, police opened the van and discovered 13 Molotov cocktails - firebombs made from wine bottles, filled with lighter fuel, perhaps intended for another wave of attacks.
Butt (27) had wrapped the handle of his knife with duct tape and tied it to his wrist with a leather strap.
Identical knives, with the brand name Ernesto, were carried by Rachid Redouane (30) and 22-year-old Youssef Zaghba.
Commander Dean Haydon, head of Scotland Yard's Counter Terrorism Command, said: "We've been working round the clock to understand what these men did in the lead up to the attack, but we need to know more about these unusual knives.
"Where have they come from? Where might the attackers have bought them? If you know anything about these particular types of knives, please let us know. It could prove a crucial lead in our investigation."
Police have discovered the gang used a bedsit - in Barking, east London - as a safe house, rented by Redouane in April.
There they found evidence that linked it to the attack and an English-language copy of the Koran, open at a page describing martyrdom, said Commander Haydon.
Among the evidence were plastic water bottles, like those used to create the fake suicide belts each of the terrorists was wearing during the attack.
The new details come as police made two further arrests in connection with the attack, bringing the number to seven.
A 28-year-old man was arrested at around 2.00am on Saturday in Barking on suspicion of preparing acts of terrorism.
Earlier on Saturday, a 27-year-old man was arrested during a raid in Ilford, east London. He was also held on suspicion of preparing terrorist acts.