The attempted truck hire failed because Butt failed to provide payment details.
Pictures of the pink-bladed knives which the London Bridge terrorists used to slaughter innocent pedestrians have been released by police.
"It looks as if it is pretty much a contained plot involving the three of them, which is supported by the forensic evidence we've got back so far", Haydon said.
"My view at the moment is that he then went to plan B and ended up hiring the van instead".
Butt drove the van into pedestrians on the iconic bridge.
A Met Police spokesperson told CNN that an investigation into Butt started in the summer of 2015, during which police received a call to the anti-terror hotline.
"I do believe from hearing other neighbours he was quite rough handled in his arrest, and dragged across the road".
The truck attacker Khuram Butt's first sought to rent was a 7.5 tonne truck 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.
The van was used to mow down the first victims, killing three of them, as it mounted the pavement on the busy bridge last Saturday night.
Soon after, police opened the van and discovered thirteen Molotov cocktails - firebombs made from wine bottles, filled with lighter fuel, perhaps intended for another wave of attacks.More news: Angela Merkel demands quick Brexit negotiations after United Kingdom election
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"Where have they come from?"
"We will be looking at intelligence and our processes, and asking ourselves the question: 'Could we have prevented such an attack?'", Haydon said.
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.
Although Islamic State in Iraq and Syria (ISIS) militants have claimed responsibility for the attack, Commander Haydon said there was no evidence that the attackers - Pakistani-born Briton Khuram Butt, Italian Youssef Zaghba and Rachid Redouane who had links to Libya, Morocco and Ireland - were directed by anyone else, either in Britain or overseas.
Mr Haydon said the belts were tested by explosives officers who found they were not viable.
Police say they have interviewed 262 witnesses, from 19 countries.
Saturday's attack - the third in three months in which the majority of the suspects had been on the radar of security officials - has prompted Prime Minister Theresa May to call for tougher counterterrorism laws even if it means changing human rights protections.
The Moroccan-born Italian attacker was briefly detained in Bologna past year, presumably while trying to reach Syria through Turkey to join the ranks of Islamic State (IS, formerly ISIS/ISIL) terrorists.
Collina said she had spoken to Italy's anti-terrorism police a year ago after her son was prevented from travelling to the Middle East, and that they were the ones to told her on Tuesday that her son had been one of the men who carried out the attack.
"You will be seeing police, armed and unarmed, in areas where there are crowds and around the streets of London, possibly at a higher level than you have done before". There were also craft knives, phone chargers, bottles of superglue, pieces of rubber and plastic and rolls of duct tape.
A total of 18 people have been arrested since the attack, with five remaining in custody.