The man who carried out the stabbing attack at London Bridge was a former prisoner convicted of a terrorism offence, the BBC has learned.
The attacker was out of prison on licence at the time of the attack, in which two people were killed and three were injured, sources said.
The man was shot dead by officers after members of the public confronted and restrained him.
Police have declared the attack a terrorist incident.
Met Police Commissioner Cressida Dick told a press conference the attack began at Fishmongers’ Hall, at the north end of the bridge, where a criminal justice event was taking place.
Dozens of people were in attendance at the event, including students from Cambridge University and former prisoners.
The university said it was “gravely concerned” about reports of its students and staff being caught up in the attack and was urgently seeking further information.
Ms Dick said officers confronted the suspect – who was found to be wearing what is believed to be a hoax explosive device – within five minutes of receiving the initial call.
She said police were working at “full tilt” to understand what had happened and whether anyone else was involved.
Officers were still working to identify the dead, she said.
Of the injured people, one is in a critical but stable condition, a second is stable and a third has less serious injuries, the head of the NHS, Simon Stevens said.
Details of what happened are still emerging, but videos on social media appear to show passers-by holding down the suspect on London Bridge.
Another man in a suit could be seen running from him, having apparently retrieved a large knife.
Footage then shows an officer arriving, seeming to indicate to the group to move, and firing a shot.
One witness described how a man at the event at Fishmongers’ Hall grabbed a narwhal tusk – a long white horn that protrudes from the porpoise – that was on the wall, and went outside to confront the attacker.
The actions of the public have been widely praised, including by Prime Minister Boris Johnson, Labour leader Jeremy Corbyn and Ms Dick, who said they had shown “extreme courage”.
She added: “Ours is a great city because we embrace each other’s differences.
“We must emerge stronger still from this tragedy. In doing that we will ensure that the few who seek to divide us will never, ever succeed.”
As details emerge about the suspected attacker, the more this looks like it was a targeted incident rather than the random attack on members of the public seen two years earlier at London Bridge.
Authorities were able to identify him quickly after it took place. Their priority since has been to understand what motivated him, whether he had any wider help and if there is anything that poses a residual threat.
There was no advance warning or intelligence of an attack but the details of the man and his past may well raise questions about whether enough has been done to monitor and understand the risks posed by former prisoners.
Earlier, Neil Basu, head of UK counter-terrorism policing, said the suspect was wearing what is believed to be a hoax explosive device.
BBC security correspondent Frank Gardner said attackers wear fake suicide vests for a couple of reasons: To add to public panic and in some cases to ensure police shoot them dead.
To some attackers, martyrdom is preferable to capture, trial and spending many years in prison, our correspondent added.
Police have said cordons will remain in place and extra police patrols will be held across the capital.
The prime minister, who has returned to Downing Street from his constituency, has been convening a meeting of the government’s emergency Cobra committee.
He did not talk about the identity of the attacker, but said he had “long argued that it is a mistake to allow serious and violent criminals to come out of prison early”.
Mr Johnson added that the emergency services and members of the public who intervened “represent the very best of our country”.
“This country will never be cowed or divided or intimidated by this sort of attack and our values, our British values, will prevail,” Mr Johnson added.
London Mayor Sadiq Khan thanked the members of the public who showed “breathtaking heroism” in running towards danger – “not knowing what confronted them”.
‘Get off and run’
Witnesses have spoken to the BBC about what they saw.
Amanda Hunter, who was on a bus on London Bridge at the time, said: “All of a sudden [it] stopped and there was some commotion and I looked out the window and I just saw these three police officers going over to a man…
“It seemed like there was something in his hand, I’m not 100% sure. But then one of the police officers shot him.”
Bus driver Mustafa Salih, 62, was travelling from Borough High Street towards London Bridge where he saw emergency vehicles and the police cordon.
He told BBC London: “A police officer came up to me and said ‘turn off your engine, get off and run’.
“I looked up and I could see a crowd of people coming towards me.
“One woman was crying. I ran back down to Borough High Street. It was all very scary as we did not know what was happening.”
London Bridge was the scene of another attack, on 3 June 2017, in which eight people were killed and many more injured.
This latest attack comes after the UK’s terrorism threat level was downgraded on 4 November from “severe” to “substantial”, meaning that attacks were thought to be “likely” rather than “highly likely”.
The terror threat level is reviewed every six months by the Joint Terrorism Analysis Centre, which makes recommendations independent of government.
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