Butt, who police consider the attack ringleader, had been on bail after being arrested for fraud in a case in October of previous year, police said.
At two minutes before 10pm the van crossed London Bridge heading south.
Inside the van, they found 13 supposed Molotov cocktails - made from wine bottles and an apparent flammable liquid - and two blow torches, according to the news station.
Police identified (from left) Khuram Shazad Butt, Rachid Redouane and Youssef Zaghba as the three attackers.
She said: "People will be thinking about what happened last Saturday night and respecting those who have lost their lives, their families and those who have been affected but also not giving in to terrorists and that includes in the immediate surrounds of Borough market".
Of the 20 people arrested, 12 have been released without charge.
"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.
Police said the men tied 12in (30cm) pink ceramic knives to their wrists and had petrol bombs in the van. Officers also discovered a Holy Quran in their safe house, opened at a page on martyrdom.
"My observation is that in relation to the threat that we have been facing most recently and the attacks that have been successful and those that have been unsuccessful, the majority of them have a domestic focus if I can put it that way, a domestic centre of gravity".
Met Police confirmed to CNN that Butt was out on bail at the time of the attacks, having been arrested in a low-level fraud offense in October 2016.
The police are of the opinion that there could be others who could throw some more light on the attack and want them to come forward. The other two terrorists were not known to the police.
Earlier on Saturday police said a 27-year-old man had been arrested in connection with the attack during a raid in Ilford, east London.
The three attackers were wearing fake suicide belts consisting of plastic water bottles wrapped in grey duct tape during the London attack.
The kitchen knives are branded Ernesto, police said. "We will be looking at intelligence and our processes, and asking ourselves the question: 'Could we have prevented such an attack?'", Haydon said.
"When we got there, there were a number of armed officers and there were casualties on the pavements", Inspector Cole said.
Ms Dick said the threat to London and the United Kingdom is one many other countries are facing and remained defiant when asked about the future.
Donald Hyslop, chair of trustees at Borough Market, said it was important for the "strong community" to come together to mark one week since the attack.