London (dpa) - British Prime Minister Boris Johnson has visited the scene of the fatal terrorist stabbing attack in London that resulted in the deaths of two civilians and the attacker himself.

Accompanied by Home Secretary Priti Patel, Johnson surveyed the scene late Saturday morning with Metropolitan Police Commissioner Cressida Dick and City of London Police Commissioner Ian Dyson, the Press Association (PA) reported.

A 28-year-old man, identified by authorities as Usman Khan, is suspected of having stabbed two people to death near London Bridge on Friday before being shot dead by police; he had previously been convicted of terror offences.

Three others remain in hospital with injuries, police said.

Khan, who served time for a plot to bomb the London Stock Exchange, had been released early from prison in December 2018, prompting a fierce debate about the practice. The Times newspaper reported that he was still wearing an electronic ankle tag.

The Guardian reported that the judge in Khan's case said his plans were a "serious, long term venture in terrorism" and he may represent an ongoing danger to the public.

"[It is a] mistake to allow serious and violent criminals to come out of prison early and it is very important that we get out of that habit and that we enforce the appropriate sentences for dangerous criminals, especially for terrorists," Johnson said before chairing a Friday evening meeting of the British government's emergency committee Cobra.

Responding to criticism, the Parole Board issued a statement on Saturday to say it had not been involved in his release of the attacker "who appears to have been released automatically on licence (as required by law), without ever being referred to the Board."

Condolences continued to pour in. Queen Elizabeth II sent her "thoughts, prayers and deepest sympathies to all those who
have lost loved ones" and praised the "brave individuals who put their own lives at risk to selflessly help and protect others."

More details of the attack emerged on Saturday. Commissioner Dick confirmed to journalists that the attacker is believed to have acted alone, PA reported.

Khan's attack started at a conference on rehabilitation at Fishmongers' Hall; he was tackled by ex-offenders attending it and a Polish chef who worked at the venue, PA reported.

Khan had taken part in a prisoner rehabilitation programme organized by Cambridge University and had shown "no cause for concern," PA cited a source familiar with the programme as saying.

Campaign events for the upcoming British general election on December 12 were cancelled on Saturday.