UK police are expected to name the men behind Saturday night's terror attack in London, in which seven people were killed and 48 others injured.
British Prime Minister Theresa May said police had identified all three attackers and would release their names "when the investigation permits."
Police said 11 people had been detained in a series of raids following the attack, in which three people rammed a van into pedestrians on London Bridge and went on a stabbing spree in nearby Borough Market, a bustling area full of bars and restaurants.
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. The three men in Saturday's assault were shot dead by police, who said they were wearing suicide belts that turned out to be fakes.
The police said there would be "increased physical measures on London's bridges to keep the public safe," and on Monday, concrete barricades had been erected on at least one of the city's major bridges as commuters made their way to work.
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 they had seized a huge amount of forensic material in two early morning raids in east London Monday, as part of their efforts to determine if the three men who carried out Saturday's attack were part of a wider network.
Officers said late Sunday night that they will release the identities of the attackers "as soon as operationally possible."
Details have 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.
ISIS claimed responsibility for the attack on Sunday night, although they provided no evidence for their involvement or details of the attack. ISIS routinely claims attacks that it has no links with.
PM Theresa May: 'Enough is enough'
Of the seven people killed, one has been identified as Canadian woman Chrissy Archibald, who had moved to Europe to be with her fiance.
Another 36 men and women are currently in hospital, 21 of whom are in a critical condition, according to NHS England.
A memorial is planned for 6 p.m. Monday in the capital's Potters Field Park, as the city continues to digest its second attack in two months.
London Mayor Sadiq Khan said the city's "anger and grief" was indescribable.
"We are all shocked and angry today but this is our city. We will never let these cowards win and we will never be cowed by terrorism," he said in a statement.
UK Prime Minister Theresa May pledged a tougher attitude to fighting homegrown extremism in a speech Sunday, outside her official residence at 10 Downing Street.
"Enough is enough. Everybody needs to go about their lives as they normally would ... But when it comes to taking on extremism and terrorism, things need to change," she said.
Neighbor saw attacker teaching children to pray
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. He was standing over there, I could see them from my window," Gasperri said.
Ikenna Chigbo, in an interview with Britain's Independent Television News, described another of the arrested men as a nice guy who regularly invited neighbors to barbecues and played football and table tennis with them.
"Yesterday -- I'm actually in the process of moving home at the moment -- I hired a van moving some bits. He came to me. He was a little bit overnice," Chigbo told the station.
"He said to me, 'Where can I get a van like that?' Asking me all the details like how much was it, where he could get a van -- basically because, he said to me, 'I might be moving shortly with my family as well.'"
Three days until election
The rush to dismantle the London Bridge attackers' network comes just days before the UK is scheduled to go to the polls.
Voting in the general election will begin at 7 a.m. on June 8, local time, to decide whether UK Prime Minister May will be returned as the country's leader.
The Conservatives, Labour and the Scottish National Party all suspended campaigning Sunday for the second time this election.
All parties had previously canceled events after the bombing in Manchester outside an Ariana Grande concert in May.
Speaking at Ford Theater in Washington Sunday night, US President Donald Trump described the incident as a "horrific terrorist attack."
"The bloodshed must end. The bloodshed will end. As president I will do what is necessary to prevent this threat from spreading to our shores," he told reporters.
Trump added he had pledged the United States' "unwavering support" to Prime Minister May following the attack.