The three London Bridge attackers had planned to hire a 7.5 tonne lorry to carry out last week's attack, British police said Friday, however they failed to provide payment details.
The men opted for a smaller Renault van instead, Scotland Yard said, which they then used to mow down pedestrians on the bridge before stabbing people in Borough Market.
The attackers used 30 centimetre pink ceramic knives with duct tape wound around the handles, before they were shot dead by police.
Police Commander Dean Haydon, head of the Met's Counter Terrorism Command, appealed to the public for information about the knives.
"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," he said.
"Where have they come from? Where might the attackers have bought them from? If you know anything about these particular types of knives, please let us know - it could prove a crucial lead in our investigation," Haydon added.
Investigators found Molotov cocktails and two blow torches in the back of the van.
Haydon said counter-terrorism officers have so far spoken to 262 people from 19 different countries. "Of these, 78 are significant witnesses."
Commander Haydon continued: "Many witnesses we've spoken to have provided extremely helpful accounts of the attack but we are still missing pieces of the picture."
At total of eight people died in the attack, three of whom were killed by the van and five who were stabbed to death, investigators said.
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