The three attackers were wearing fake suicide belts consisting of plastic water bottles wrapped in gray duct tape.
Khuram Butt, Rachid Redouane and Youssef Zaghba killed eight people last Saturday night after driving into pedestrians and then embarking on a stabbing spree on the popular nightlife spot.
Twelve other people who were detained in the wake of the van and knife attack in London were released without charge.
Police are questioning six men over suspected links to the attackers, Pakistani-born British citizen Butt, Moroccan Redouane, and Zaghba, an Italian national of Moroccan descent. "These knives aren't mainstays, so clearly you can get a better sense of who's selling them and for what objective, so it could be useful in the actual investigation itself".
On Saturday, Commander Dean Haydon of the Met's anti-terrorist command said: "Many witnesses we've spoken to have provided extremely helpful accounts of the attack but we are still missing pieces of the picture".
The intended truck was smaller but similar to the one used in the Nice attack past year that killed 86 people and injured hundreds in the resort town in the south of France.
Fake suicide vests worn by the assailants in the London Bridge attack were made with disposable water bottles attached to leather belts, police revealed Sunday.
United Kingdom police warn use of fake explosive belts is 'new tactic'
A 28-year-old man was arrested at around 2am on Saturday in Barking, east London , on suspicion of preparing acts of terrorism . Anyone with information can call the police in confidence on 0800 789 321 or visit gov.uk/ACT and complete a secure form.
He said it was hard to speculate the exact reasoning behind the belts, but they may have been meant to take hostages or as a defensive strategy.
"It makes the bravery of those police officers and members of the public who tackled the terrorists even more remarkable", he added. They killed eight people before being shot dead by police.
Also in the van were 13 Molotov cocktails and two blow torches to ignite them - which could also fit the siege theory.
Police said the death toll could have been higher if Butt had managed to rent the larger 7.5 tonne lorry, but his payment details failed to go through.
The van also had office chairs and a suitcase.
After Butt's payment was declined, the trio of terrorists rented the smaller van that they used to plow into crowds before jumping out of the vehicle and going on a stabbing rampage that left eight people dead and nearly 50 injured.
'On the radio, I could hear officers up at London Bridge and outside the Mudlark [pub] who were doing CPR and they were calling for ambulances urgently.
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