Khuram Shazad Butt, 27, was a British citizen who was born in Pakistan.
But with Britons due to vote in a national election on Thursday, her decision to reduce the number of police officers in England and Wales by nearly 20,000 during her six years as interior minister from 2010 to 2016 shot to the top of the political agenda.
As details about Butt emerged, however, they prompted questions of whether he could have been stopped sooner.
He had appeared in a documentary, "The Jihadis Next Door", that aired on British television previous year. All three have been claimed by the Islamic State group. "This is a passing time for us".
He was not known to the police. "He told me, 'I will do anything for my religion, ' " she said, referring to Butt. "I got very angry".
Butt briefly worked for Transport for London, the entity that oversees the city's public transport system.
Ignacio Echeverria, 39, remains missing since Saturday attack. Rachid Redouane, 30 had claimed to be Moroccan and Libyan. No arrests had been made as part of the search, the force said.
London Mayor Sadiq Khan stands side-by-side with the chief of London police, Cressida Dick, to condemn attack at London Bridge, in which seven people were killed. The three men, who wore fake suicide vests, were shot dead by the police.
All three perpetrators were confronted and shot dead by armed officers within eight minutes of the first call. Twelve others taken into custody have since been released.
Prime Minister Theresa May has said Britain is faced with a new trend where "terrorism breeds terrorism" and inspired not just by "carefully-constructed plots after years of planning and training - and not even as lone attackers radicalised online - but by copying one another and often using the crudest of means of attack".
"I think Sadiq Khan is doing a good job and it's wrong to say anything else - he's doing a good job", she told a press conference on Monday.
"There's an election on Thursday, that's the chance", he said.
DANIEL SORABJI via Getty Images People have commended police and paramedics for their'heroic work in the wake of the terror attack
Last month Butt was seen urging people in east London not to vote in the general election, The Guardian reported, while last year he had appeared in Channel 4 documentary Jihadis Next Door last year alongside known radicals.
"You have to have a big heart, a lot of courage, a lot of compassion to put your time and efforts into working in this sector", she said. The victims reportedly included British nationals and foreigners.
Under the British government's counterterrorism program, residents are encouraged to alert police to suspicious activity. The matter can be dropped or if the complaint seems warranted, police and security officials can open an investigation.
This story will be updated as new information becomes available.
Jeremy Corbyn, leader of the main opposition Labour Party, called on May to resign over the issue, saying earlier that "you can not protect the public on the cheap", and promising to recruit another 10,000 police officers if he is elected into power.
Some 20,000 other suspects, however, are on the periphery, according to the sources.
It was lowered once intelligence agencies were comfortable this wasn't the case.
British police clear the scene of the attack near London Bridge, loading the van used onto a truck.
After the auto crashed into the perimeter fence of the Palace grounds, Masood abandoned it and ran into New Palace Yard where he fatally stabbed an unarmed police officer. At least 12 persons were arrested on Sunday, including five men and seven women, ranging in age from 19 to 60.
The president and the mayor's feud goes back to the presidential campaign and when Khan was first elected in May 2016.
Mr Khan declined to respond but the president returned to the attack on Monday, after the media pointed out that he had misrepresented the mayor's remarks.
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