The London Bridge attackers tried to hire a seven-and-a-half tonne lorry to carry out their attack, say police as two more people have been arrested in connection with the atrocity.
The three men failed to provide payment details and the vehicle was not picked up, which led to them using a smaller van from a DIY store instead.
The men drove into pedestrians on the bridge before stabbing people in Borough Market on June 3.
Police said the men tied 12in (30cm) pink ceramic knives to their wrists and had petrol bombs in the van.
Eight people were killed and dozens more were injured in the attack, which began shortly before 10pm
Khuram Shazad Butt, Rachid Redouane and Youssef Zaghba have been named as the attackers, who were all shot dead.
Scotland Yard wants more witnesses to come forward and is also appealing to companies who hire out vans to report any suspicious activity.
Early this morning, police said they had arrested a man, 27, on suspicion of preparing terrorists acts, after carrying out a raid in Ilford, east London, linked to the attack.
At 2.05am, police also arrested another man, 28, at a residential address in Barking, also on suspicion of preparing terrorists acts.
In its most detailed description of the attack yet, the Metropolitan Police said the men had also rented a flat in Barking, east London, to use as a safe house.
Inside, they found a copy of the Koran opened at a page describing martyrdom.
They also discovered equipment for making petrol bombs, plastic bottles and duct tape for constructing fake suicide bomb belts, and an ID card for Redouane.
Forensic work at this flat has determined that they acted alone.
The investigation has concluded that 27-year-old Butt was the ringleader. He hired the van used in the attack from a B&Q store in Romford on the morning of the attack.
He is thought to have driven the van, with Redouane and Zaghba in the back, into central London.
At two minutes before 10pm the van crossed London Bridge heading south. Six minutes later it returned, crossing over the bridge again and making a U-turn at the northern end.
The attack began with the van driving back along the pavements of the bridge, running down three pedestrians before crashing outside the Barrowboy and Banker pub opposite London Bridge station.
The men jumped out clutching the ceramic kitchen knives. Police say the weapons were possibly chosen to prevent them being picked up by metal detectors.
They stabbed five people who had been enjoying the area's pubs and restaurants.
Police were called within two minutes and arrived eight minutes later, killing the attackers in what officers said was an 'unprecedented' volley of 46 bullets.