Updated 3.05pm - The 19-year-old suspect arrested after a knife rampage in central London last night is a Norwegian national of Somali origin, Scotland Yard has revealed.
Assistant Commissioner Mark Rowley said that so far no evidence has been found of radicalisation that would suggest the man in custody was "in any way motivated by terrorism".
It was also revealed that the woman killed in the attack in Russell Square was American. Those injured are Australian, American, Israeli and British.
No Maltese were among the casualties.
Mr Rowley said specialist detectives from murder and terrorism teams had been working on the case throughout the night.
He went on: "Whilst the investigation is not yet complete, all of the work we have done so far increasingly points to this tragic incident as having been triggered by mental health issues.
The investigation so far increasingly points to this being a tragic incident triggered by mental health issues.
"At this time, we believe this was a spontaneous attack and that the victims were selected at random."
Investigators have made "urgent progress" in five strands of work, he said - interviewing the suspect, speaking with his family, witness accounts, address searches and a "full intelligence review" by police and the security service.
This morning officers searched an address in north London and will search another in south London.
Mr Rowley, Britain's most senior counter-terrorism officer, said: " I emphasise that so far we have found no evidence of radicalisation that would suggest the man in our custody is in any way motivated by terrorism.
"The suspect is a Norwegian national, of Somali origin. I stress though that all the work we have done so far does not suggest that those factors in his background are relevant to the motivation for his actions."
The suspect emigrated from Norway in 2002, a spokesman for the country's Ministry of Foreign Affairs said.
The rampage unfolded in a popular tourist area near the British Museum in Bloomsbury.
It is around a mile north of the bustling area around the Strand, where several theatres and scores of restaurants and bars are located.
Armed officers raced to the scene shortly after 10.30pm oafter reports of a man attacking people with a knife.
Mr Rowley said: " Within six minutes of that call armed officers were there. The man was Tasered and he was detained.
"Our firearms officers bravely rushed to that scene with very little information. They would not have known if this was an act of terrorism or a random attack we now know it to be.
"They detained an armed and dangerous man - and resolved it using the minimum amount of force - no shots were fired."
A woman, believed to be in her 60s, was pronounced dead at the scene, while two other women and three men were injured.
Two remain in hospital while the others have been discharged. None is in a life-threatening condition.
London mayor Sadiq Khan called for the public to remain "calm and vigilant".
A cyclist who was passing the scene moments after the stabbing told how he was flagged down for help by a Spanish family.
Fernando, 40, from Brazil, said he was the first to call the ambulance and saw an "older woman" slumped against the railings with a stab wound to her back.
He added that he had not seen the attacker but said the Spanish family told police the man was wearing black shorts and a white shirt, and was covering his face as he made sweeping, stabbing motions.
Fernando went on: "The moment the police arrived, they asked them if the man had been shouting. They said that he didn't say anything. They said that when he stabbed the people he didn't shout or scream anything."
No Maltese involved in the attack - High Commissioner
The area where the attack took place is very popular with Maltese tourists.
Malta's High Commissioner in London, Norman Hamilton, said when contacted that the Metropolitan Police confirmed to the High Commission there were absolutely no Maltese involved in the attack.
Derek Fenech, who is in London staying at The Royal National Hotel, which is alongside Russel Square, said he and his wife had passed by the area an hour or so before the attack.
At around 10pm, he heard commotion and screams which he ignored. Some 30 minutes later, a large number of sirens, ambulances and police cars passed by. At this point, he was still thinking that this was part of the urban sound of London.
But watching the news at around 2am, he soon realised what had actually happened.
Mr Fenech said the area was this morning patrolled by armed police and a helicopter was hovering above. He said that police presence around museums in London was heavy yesterday even before the attack.