British police have named the third London attacker as 22-year-old Youssef Zaghba, an Italian of Moroccan descent from east London.
The Metropolitan police said he was not a "person of interest" before Saturday's attack.
Earlier police admitted that one of the other attackers was on their radar as a member of an outlawed radical Islamist group, but said an investigation into him had been downgraded as there was no evidence he posed an imminent threat.
Khuram Shazad Butt was associated with the banned extremist group al-Muhajiroun, co-founded by notorious hate preacher Anjem Choudary, and appeared in a 2016 documentary called "The Jihadis Next Door."
Assistant Commissioner Mark Rowley of London's Metropolitan Police acknowledged that Butt, a British national born in Pakistan, was known to authorities. "However, there was no intelligence to suggest that this attack was being planned and the investigation had been prioritized accordingly," he said.
Butt was one of three men who rammed a van into pedestrians on London Bridge in the city's center before launching a stabbing spree in bars and restaurants at the nearby Borough Market. Seven people died and 48 were injured.
Questions have swirled over why police were unable to do anything about Butt. And just two days before the UK election, Prime Minister Theresa May has faced a barrage of criticism for cutting 20,000 officers from the police force in her time as Home Secretary.
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. The probe continued but was moved into "the lower echelons of investigations."
"Looking back over the information we had at the time, so far, there is nothing to show that poor decisions were made. We will probably discover communications during that period that we couldn't access that would have change the situation," the spokesperson said.
UK Foreign Secretary Boris Johnson said that the country's intelligence agencies had questions to answer.
"People are going to look at the front pages today and they're going to say, 'How on earth could we have let this guy or possibly more through the net?'"
Twelve people were arrested after the attack but have all now been released without charge, the Met Police said on its Twitter account Monday. It also reported that officers had raided an address in Ilford, east London, in the early hours of Tuesday morning.
Butt is a 27-year-old British citizen who was born in Pakistan. A second attacker was named as Rachid Redouane, a 30-year-old who had claimed to be Moroccan and Libyan.
Butt, who also used the name Abdul Zaitun, appeared in a 2016 documentary called "The Jihadis Next Door," which told the story of Abu Rumaysah, the Londoner who has appeared in recent ISIS propaganda. At one point in the documentary, Butt can be seen unfurling a black banner in Regent's Park.
A London mosque said Butt "infrequently" attended prayers there but was asked to leave after an incident a few years ago where he interrupted a Friday sermon, according to a statement from Jabir bin Zayd Islamic Centre.
In two videos viewed by CNN, Butt is among a group of men searched by police in 2015 after a member of the public reported seeing the group praying in a park with what appeared to be an ISIS flag.
The ISIS-linked Amaq News Agency claimed a "detachment of Islamic State fighters" carried out the attack but provided no evidence to back up its claim.
May under fire
Despite a rapid response to Saturday's deadly attack, which saw armed officers shoot dead all three suspects within eight minutes, opposition parties have called for May's resignation for her cuts to police numbers.
Asked if he backed calls for May's resignation just days before the country's General Election, main opposition Labour Party leader Jeremy Corbyn told ITV, "indeed I would."
He later clarified: "I think we should vote on Thursday to decide who our MPs are and who are government is going to be ... There's an election on now, there's a choice for everybody. There's a deep anger from those people who've seen 20,000 police officers lose their jobs."
Corbyn's party colleague, London Mayor Sadiq Kahn, told Britain's Channel 4 News that the police force is underfunded.
"We simply don't have the resources we need to provide the best service we can," he said in an interview following Monday's vigil.
Since 2010, when the Conservatives entered government, the number of police officers has fallen from 145,948 to 126,766 in March 2016, according to Home Office figures cited by ITV.
Liberal Democrat leader Tim Farron added his voice to the chorus, saying that, in her former role and now as Prime Minister, May had "shown contempt for our police force."