The suspect in a knife rampage in central London 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 on Wednesday night was American. Those injured are Australian, American, Israeli and British.
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.
“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.
On Thursday 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.
Police said on Thursday morning that a 19-year-old man had been arrested on suspicion of murder following the incident 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 on Wednesday after 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.
The events unfolded hours after Scotland Yard announced that more armed officers would be deployed on public patrol around the capital after a spate of attacks in mainland Europe.
Asked what led police to say in the early hours of Thursday that terrorism was a line of inquiry, Mr Rowley said: “You would expect us in the current climate of threat and with the events across Europe, when we have a violent attack like this with multiple victims, to consider all possibilities.”
He said police were “considering many lines”, adding: “We have made substantial progress and our focus is narrowing.”
Mr Rowley appealed to anyone in the area at the time to contact police.
He added: “This attack will of course still cause concern and as a precautionary measure we have increased the number of officers out on the streets today and that will continue for as long as necessary.”
Met Police Commissioner Sir Bernard Hogan-Howe vowed to protect London, adding: “A normal night out in our busy capital has ended in horrific circumstances.”
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.”
Jodie Parry, who was in her hotel room when she heard a “commotion” in the street, told the BBC that the attacker ignored police as they screamed at him to stop running.
She added: “He was actually carrying a knife in his hand and he had blood on his hands.”
Manuel Simo, 32, told the Evening Standard: “At first, people were confused and thought it was a traffic accident.
“When they realised what was happening, people were screaming and running away from a guy who was holding a knife.”
Michel Godbout, who was staying at the President Hotel close to Russell Square, said he heard a woman screaming around the time of the attack.
Mr Godbout, who is visiting from Canada, told the Guardian: “About 10.30 I was in my room and I heard a woman screaming - a couple of times in one minute - then I didn’t hear anything.”
The events unfolded close to Tavistock Square - the scene of the bus bombing during the July 7 attacks in 2005.
Pol O’Geibheannigh, 45, was walking back from the cinema when he saw a massive police presence and a woman lying in a pool of blood.
He said: “That level of police presence I haven’t seen since 7/7 and I was here - I live around the corner.”
As the day went on, members of the public and representatives of local businesses lay floral tributes at the scene where the American woman was killed.
No personal messages were attached to the flowers, some of which carried general messages of condolences.