Butt, 27, was a British citizen who was born in Pakistan, London's Metropolitan Police said. Police and MI5, the UK's counterintelligence and security agency, were familiar with him, but there was no intelligence to suggest the weekend attack was being planned, police said.
Butt briefly worked for Transport for London, the entity that oversees the city's public transport system. He was a trainee customer services assistant with London Underground for less than six months, before leaving in October, a Transport for London spokesperson said.
Redouane, 30, who also used the name Rachid Elkhdar, had claimed to be Moroccan and Libyan.
The third attacker has been identified, police said, but his name has not been publicly released.
The assailants killed seven people and wounded 48 others by ramming a vehicle into a crowd on London Bridge and then stabbing people in nearby Borough Market.
Police shot and killed all three attackers, who were wearing suicide belts that turned out to be fakes.
Twelve people were arrested after the attack. On Monday, Met Police said all those arrested had been released without charges.
-- One attacker had a connection to Ireland, a source briefed by an Irish counterterrorism official said.
-- British Prime Minister Theresa May faced questions over cutting 20,000 police posts in her time as Home Secretary.
-- Britain's most senior police officer Cressida Dick said police resourcing needed to be revised.
-- Leader of the main opposition Labour Party, Jeremy Corbyn, called on for the Prime Minister to resign over cuts.
British Prime Minister Theresa May faced a barrage of questions Monday on deep cuts made to police numbers in the UK while she was Home Secretary.
Authorities also faced scrutiny over what they knew about the alleged perpetrators of the attack.
The carnage is the third such terror attack on British soil in three months and the second in London involving the use of vehicles as weapons.
In March, 52-year-old British national Khalid Masood rammed a vehicle into a crowd on Westminster Bridge and stabbed a police officer, in an attack that left five people dead.
Police said there would be "increased physical measures on London's bridges to keep the public safe." On Monday, concrete barricades had been erected to separate pedestrians from vehicles on some of the city's major bridges.
May under fire over police cuts
Security had already become a battleground issue ahead of Thursday's general election since a Manchester bombing last month, in which 22 people were killed as they left an Ariana Grande concert.
Pressure on May intensified Monday, when at a campaign event she faced a volley of journalists' questions over police cuts. May said her record was sound, saying she had introduced a raft of anti-terror legislation as Home Secretary. She also said that funding for counterterrorism efforts had been protected.
Metropolitan Police Commissioner Cressida Dick said Monday that the country needed to rethink police resourcing and strategy.
But May defended her government's stance on the issue, saying that her Conservative government had protected police budgets.
"The commissioner of the Metropolitan Police has said the Met is well resourced, and they are, and that they have very powerful counterterrorism capabilities, and they do. We have protected counterterrorism policing budgets. We have also provided funding for the increase of the number of armed police officers," she said at a campaign event.
Jeremy Corbyn, leader of the main opposition Labour Party, called on May to resign over the issue, saying earlier that "you cannot protect the public on the cheap," and promising to recruit another 10,000 police officers if he is elected into power.
One of the victims in Saturday's attack was identified as Chrissy Archibald, a Canadian who had moved to Europe to be with her fiance.
Another 36 men and women are hospitalized, 18 of whom are in a critical condition, according to NHS England.
A memorial was held Monday evening in the capital's Potters Field Park, as the city continues to digest its second attack in two months.
London Mayor Sadiq Khan said Monday the actions of the attackers were perverse and poisonous, adding that they had no place in Islam.
"The acts of these three men on Saturday night was cowardly, was evil and I'm angry and furious that these three men are using to justify their actions the faith that I belong to," he said.
On Sunday night,, ISIS claimed responsibility for the attack, although the group provided no evidence for its involvement or details of the attack.
Attacker was once asked to leave East London mosque
Butt "infrequently" attended Jabir bin Zayd Islamic Centre in Barking, that mosque said. "We did not know him well, his name was not known to us. It has been brought to our attention that some years ago after interrupting a Friday sermon he was asked to leave the mosque," the statement said.
Butt, who also used the name Abdul Zaitun, is believed to have associated with the outlawed radical Islamist group al-Muhajiroun, co-founded by notorious hate preacher Anjem Choudary. Al-Muhajiroun has been linked to Michael Adebolajo, one of the men convicted of slaying British soldier Lee Rigby near a military barracks in southeast London in 2013.
Mohammed Shafiq, chief executive of the Ramadhan Foundation, said Butt verbally assaulted him the day after Rigby's death.
"Butt called me a 'Murtad,' which means traitor in Arabic, and accused me of being a government stooge when I confronted Anjem (Choudary) about him supporting terrorism and my public campaign against Lee Rigby's murder," Shafiq said in a statement Monday.
Shafiq said police arrived, and Choudary, Butt and two other men were escorted away.
"It is clear that Anjem (Choudary) and his band of terrorist sympathizers were known to the police for many years," Shafiq said.
He added: "I am not surprised that Khuram Butt carried out the terrorist attack and there are serious questions for the authorities."
Neighbor saw attacker teaching children to pray
Police said they seized a huge amount of forensic material in two early morning raids in east London on Monday, as part of their efforts to determine if the three men who carried out Saturday's attack were part of a wider network.
Details of the investigation had been kept closely under wraps, in stark contrast to last month's Manchester bombing, when photos and information from the investigation were repeatedly leaked to the US media, triggering a row between the British and American governments.
Monday's early morning raids follow raids on a housing complex in Barking east London on Sunday, where a series of arrests were made.
CNN's Melissa Bell spoke to residents there who recognized a familiar face among the three dead attackers, identifying him as one of their neighbors and describing him as a family man who kept to himself.
Barking resident Erica Gasperri said she went to the police after she saw a man, believed to be the attacker, teaching the local children about Islam.
"All of a sudden we saw this individual speaking to the kids ... showing them how to pray," she said. "I could see them from my window."