"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", said Commander Dean Haydon, head of the Metro Police Counter Terrorism Command.
More than 200 Muslim leaders signed a statement condemning the attack and announcing that they would refuse traditional Islamic prayers for the perpetrators and called on others to do the same. "They are a diverse bunch", he said.
Counter-terrorism officers have so far spoken to 262 people from 19 different countries; of these, 78 are significant witnesses.
In a rare glimpse into the weeklong investigation, police released details on Saturday that showed Butt originally tried to rent a 7.5-tonne truck. It was not clear why he could not pay, or if he lacked the necessary licence to drive such a vehicle.
At about 1730 GMT, the men drove to pick up the van before heading to Zaghba's home in east London.
British police announced the arrest of 28-year-old man on investigation of the terrorist attack on London bridge. Tape was wrapped around the handles. Police shot the attackers dead. Their victims were three French nationals, two Australians, a Canadian, a Spaniard and a Briton.
Petrol bombs were also found littered alongside the van used to run down victims on London Bridge, suggesting the plot was meant to be more destructive.
In an image of the white rented vehicle, at least two of the crude exploives could be seen at the door of the passenger seat, alongside two blow torches. "They were still fairly close to the van".
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, it said.
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Detectives believe the gravel may have been placed in the vehicle to make it heavier, or as part of a cover to justify hiring it, while the chairs may have been used to convince family and friends they were moving furniture.
In the room that was rented by one of the murdered militants - Rachid Redouane - the police found two empty five-liter bottles that smelled of gasoline, rolls of black and gray scotch tape, belts and an English-language edition of the Kuran, opened to a page describing martyrdom.
Haydon said since last Saturday they had taken 262 statements from people from 19 different countries and numerous worldwide inquiries were ongoing relating to the attackers and the victims. Haydon acknowledged that Butt had links to al Muhajiroun, a banned group headed by cleric Anjem Choudary.
Butt had been on bail after being arrested for fraud in a case in October, police said.
The aim of the attackers seems to have been to replicate the attack in Nice, the officer said.
"Obviously we will be reviewing, for example, the number of armed officers we have and how they work, and a whole host of other things that are likely, from my point of view, to take more resources, and I will be asking for them".
The Metropolitan Police said the men had rented a flat in Barking to use it as a safe house.
Security staff from the market will help residents access the closed areas, which can be entered via the Stoney Street junction with Borough High Street.
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