London Bridge attackers tried to rent lorry ahead of deadly attack
Khuram Butt, 27, was unable to rent a 7.5-tonne vehicle due to payment issues, leading the trio to hire a white van before driving into pedestrians on the bridge and launching a stabbing rampage in the market.
Head of the Metropolitan Police's Counter Terrorism Command, Dean Haydon, said "the effects could have been even worse," if the attackers had managed to rent a larger vehicle.
Eight people have died and 48 others were injured in last Saturday's attack, which saw the assailants being shot and killed by police.
The men used 30 centimeters (12 inches) pink ceramic knives strapped to their wrists to carry out the attack, according to police.
"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 from," said Hayden, in an appeal for more information.
Scotland Yard said a 28-year-old man had been arrested on Saturday morning in the Barking neighborhood of east London, where the attackers had a safe house.
"He was arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism under Section 41 of the Terrorism Act 2000," a Met statement said.
Security forces have so far made 20 arrests in connection with the terror attack, with seven remaining in custody