National counter-terrorism police chief Mark Rowley named two of the three slain assailants as Khuram Butt and Rachid Redouane, revealing that Butt had been known to security services.
The aftermath of Saturday night's rampage, which left seven dead and dozens wounded, dominated the campaign trail ahead of Thursday's general election.
Labour leader Jeremy Corbyn said he would support calls for May to quit, as she had overseen a sharp reduction in police numbers in her past job as interior minister.
All 10 people still being held as part of the investigation into the terror attack in London have been released without charge, police said on Monday.
"All of the people arrested on Sunday, 4 June, as part of this investigation have now been released without charge," London's Metropolitan Police said in a statement.
Sadiq Khan leads vigil for victims of terror attack
The attack, claimed by the Islamic State group, saw three men wearing fake suicide vests use a white van to mow down people on London Bridge and then slash and stab revellers enjoying a Saturday night in the bustling Borough Market area.
Armed police reacted swiftly, killing the attackers within eight minutes with 50 shots.
Butt was 27 and a British citizen born in Pakistan.
He appeared in a Channel 4 documentary entitled "The Jihadis Next Door" about British extremists that was broadcast last year, local media reported.
Redouane was 30 and "claimed to be Moroccan and Libyan". Police said he also used the name of Rachid Elkhdar and a different date of birth that gave his age as 25.
Both men lived in Barking, an ethnically diverse part of east London where police carried out several raids on Sunday and Monday. Ten people are currently in custody.
Police chief Cressida Dick said investigators had seized "a huge amount of forensic material" from the van.
"A very high priority for us is to try to understand whether they were working with anybody else," she told BBC television.
Dick and Mayor Sadiq Khan visited London Bridge as commuters returned to work after some security cordons were removed.
Hundreds of mourners turned out later Monday for a sombre vigil on nearby Tower Bridge.
"To the sick and evil extremists who commit these hideous crimes, we will defeat you. You will not win," Khan said to applause.
The Amaq news agency, which is affiliated with the IS group, said the attacks were carried out by "a detachment of fighters from Islamic State".
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A Canadian and a Frenchman were among the fatalities and citizens of several nations were among the 48 injured, including Australia, Bulgaria, France, Greece and New Zealand.
Eighteen are still in critical condition, according to health authorities.
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'Evil' ideology
May blamed "evil" Islamist ideology and vowed to crack down on extremist content online, warning that attackers were "copying one another".
She said the same ideology was behind the May 22 suicide bombing at a pop concert in Manchester that left 22 people dead, and the Westminster Bridge attack in March, which killed five.
Britain's response to the terror threat must change, she said, after the three attacks which in total left 34 dead and some 200 injured.
"We cannot and must not pretend that things can continue as they are," she said, adding there was "far too much tolerance of extremism in our country".
But May's own record came under fire from Labour, which -- according to opinion polls questioned for their reliability -- has closed the gap on her Conservatives ahead of Thursday's election.
Campaigning resumed Monday after being suspended for a day.
Corbyn noted that for six years, police numbers fell while May was in charge of security as interior minister, implementing a budget-cutting drive.
Asked by ITV television if he backed the calls for May to resign, he said: "Indeed I would."
His party is calling for a drive to hire thousands of officers for neighbourhood duties, arguing that a grassroots approach will curb crime and radicalisation.
May insisted London police were happy with their resources, while counter-terrorism budgets had been protected and the number of armed officers had increased.
Spat with Trump
Britain was already on high alert following the attack at a concert by US pop star Ariana Grande in Manchester in which seven children were among the dead.
Grande headlined a benefit concert in the northwestern city on Sunday, alongside stars including Pharrell Williams and Justin Bieber.
The national threat level was raised to maximum after the Manchester attack and troops were deployed at key public sites, but reduced to its second-highest level last weekend.
Saturday's rampage was the latest in a string of attacks to hit Europe, including in Paris, Berlin and Saint Petersburg.
Global leaders sent messages of support, though US President Donald Trump raised British hackles by attacking Khan on Twitter and enlisting the attack to defend his thwarted travel ban.
Asked about the comments, Khan told Sky News: "I just haven't got time to respond to tweets from Donald Trump."
