Three men then got out of the vehicle with large knives and attacked people at bars and restaurants in nearby Borough Market until they were fatally shot by police. Butt and Redouane both lived in Barking, east London.
Two of the attackers had already been identified.
Saturday night's rampage, in which the three men drove into pedestrians on London Bridge before stabbing people in the bustling Borough Market area, was Britain's third Islamist attack in as many months.
He was among several officers who tried valiantly to stop the attackers late on Saturday night before his colleagues arrived, reports said.
The officials spoke on condition of anonymity because they were not allowed to discuss details of the case.
Youssef Zaghba is said to have been stopped by Italian officers trying to travel to Syria previous year and added to an worldwide database of alerts about individuals, to which the United Kingdom has access.
Italy has expelled more than 40 people in the past two years who were suspected of extremist activities but for whom there was insufficient evidence to bring formal charges. "We have had to close police stations, sell police buildings, and we've lost thousands of police staff", said London Mayor Sadiq Khan, a member of the opposition Labour Party.
This undated handout photo provided by the Metropolitan Police shows Youssef Zaghba .
Less is known about Rachid Redouane, who also used the surname Elkhdar, and he is not thought to have been known to the authorities.
Leaders cast ballots as General Election voting gets under way in Scotland
Ms Swinson had held the seat for a decade before being ousted by Mr Nicolson in 2015. "Nicola Sturgeon, I hope you're listening". Conservative Scottish Secretary David Mundell told BBC Scotland: "It's becoming increasingly clear the SNP bubble has burst".
The arrests were made after police raided properties in Barking on Sunday and two on Monday in Newham and Barking.
Khuram Shazad Butt, a 27-year-old Pakistani-born British citizen, was known to the police and the MI5, British authorities said in the statement, adding that "there was no intelligence to suggest that this attack was being planned and the investigation had been prioritized accordingly".
Relatives said the 28-year-old was "loved and adored by her family, friends and boyfriend", while colleagues at Guy's and St Thomas' Hospital in central London described her as "one in a million".
The Metropolitan Police also said in a statement that specialist officers were working with families and the coroner to identify those who died in the attack.
May responded to the attack by calling for an overhaul of the strategy used to combat extremism, including a demand for greater global regulation of the internet, saying big internet companies were partly responsible for providing extreme ideology the space to develop. The issue has become a key one in the run-up to Thursday's general election.
Mohammed Shafiq, chief executive of the Ramadhan Foundation, said Butt called him a "Murtad" - traitor in Arabic - when he confronted Choudary about his support of terrorism days after the murder of Lee Rigby in 2013. The second man, Rachid Redouane, had not aroused any suspicions.
Mr Rowley said work was continuing to understand more about the attackers, "their connections and whether they were assisted or supported by anyone else".
They saw a man dragged from a house and arrested while armed officers led three women out to a neighbouring garden and told them to keep their heads down.
Comments