Police said Pakistan-born Khuram Butt, one of the three attackers, was unable to rent the large truck online due to payment issues, leading the trio to hire a white van before driving into pedestrians on the London Bridge and launching a stabbing rampage in the Borough Market on June 3, BBC reported.

At least eight persons were killed, 48 were injured and the killers were shot dead by the police.

The investigation concluded that 27-year-old Butt was the ringleader and he is thought to have driven the van, with the other two attackers in the back, into central London, police said.

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.

The men used 30 centimetres (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.

Police also released photos of the van the attackers used to mow down people on the bridge, the BBC report said.

Inside, they found 13 apparently Molotov cocktails made from wine bottles and a possible flammable liquid. There were also two blowtorches.

They found a number of office chairs and a suitcase, and believe that the attackers had told family they were using the van to move as a cover story.

Police said the three men -- Butt, Rachid Redouane and Youssef Zaghba -- had driven up and down London Bridge twice before carrying out the attack, in what appeared as a dry run.

Early on Saturday, police said they had arrested a man, 27, on suspicion of preparing terrorist acts, after carrying out a raid in Ilford, east London, linked to the London attack.

They also arrested a 28-year-old man in Barking, where the attackers had a safe house.

A copy of the Quran, equipment for making petrol bombs, plastic bottles and duct tape for constructing fake suicide bomb belts, and an ID card for Redouane were found from the house.

Security forces have so far made 20 arrests in connection with the terror attack, with seven persons remaining in custody.