Updated at 6:30 p.m. ET
London Metropolitan Police have identified two of the three attackers involved in Saturday's violent rampage on and around London Bridge, releasing photos of Khuram Shazad Butt and Rachid Redouane and asking the public for help in tracing their movements.
Police shot and killed the three attackers within eight minutes of the initial emergency call about the attack that killed at least seven people. Detectives are still working to identify the third attacker.
Both Butt and Redouane had been living in Barking, east London, police say. The department adds that while Butt "was known to the police and MI5," the agencies had no reason to believe he was involved in planning the attack. Redouane was not known to the authorities, according to the police.
"I urge anyone with information about these men, their movements before the attack and where they frequented to come forward," Assistant Commissioner Mark Rowley said.
Butt, 27, "was a British citizen who was born in Pakistan," police said, while Redouane, 30, "had claimed to be Moroccan and Libyan" and had also used the name Rachid Elkhdar.
On Monday evening in London, a massive crowd gathered at Potters Fields Park, adjacent to London Bridge, to hold a vigil and pay tribute to those slain and the survivors of the attack.
Of the 48 people taken to the hospital following the violence that started on London Bridge and continued in Borough Market, officials said Monday that "36 are currently being cared for in London hospitals with 18 remaining in a critical condition."
"This was an attack on London and the United Kingdom, but it was also an attack on the free world," British Prime Minister Theresa May said Monday.
"The police are working hard to establish the identity of all those who were tragically killed or injured in the event on Saturday night," May added. "But it is now clear that, sadly, victims came from a number of nationalities."
She said the U.K.'s national threat level remained at severe on Monday, which "means that a terrorist attack is highly likely."
Metropolitan Police say that as of Monday evening local time, they had searched six properties and "arrested 12 people — seven women and five men." Later, the police tweeted that everyone arrested had been released without charges.
The Two-Way described how the attack unfolded on Saturday:
"The attack began at about 10 p.m., local time, when a van crossing London Bridge veered into pedestrians, crashing outside the Barrowboy & Banker Pub. Witnesses told the BBC that van hit 'five or six people,' then the attackers exited the vehicle and pursued others in a 'rampage' in which hundreds ran for their lives. One witness said he saw a woman stabbed '10 or 15 times' by the men, who he said shouted, 'This is for Allah.'
"The men then ran down a set of stairs and began stabbing people at a number of different pubs and restaurants in Borough Market, a historic fruit and vegetable market. Witnesses said people threw chairs and pint glasses at the men, who wore what police said were fake explosive vests."
Three Metropolitan Police officers and one British Transport Police officer were injured during the attack, the police said in a statement Monday, and one of the officers is still in serious condition.
ISIS said its "soldiers" carried out the attack. It made the claim in a statement published by its Amaq news agency.
"Questions are arising as to how much the police knew about the three attackers," NPR's Eleanor Beardsley reports from London. "One of them had been reported to police because of his extremist views and may have previously been confronted by officers."
Metropolitan Police have identified the first of the seven people killed in the attack: Chrissy Archibald, a Canadian citizen on vacation in London. Police released a statement from her family:
"We grieve the loss of our beautiful, loving daughter and sister who was 30 years old. She had room in her heart for everyone and believed strongly that every person was to be valued and respected. She lived this belief working in a shelter for the homeless until she moved to Europe to be with her fiancé. She would have had no understanding of the callous cruelty that caused her death. Please honor her by making your community a better place. Volunteer your time and labor or donate to a homeless shelter. Tell them Chrissy sent you."
The Telegraph and other news outlets reported that Archibald was struck by the van driven by the attackers and died in her fiancé's arms.