A man has been arrested on suspicion of murder after a US citizen was killed and five other people were injured in a knife attack in central London.
Police believe the attack in Russell Square on Wednesday was “spontaneous”, with victims “selected at random”.
The woman who died was thought to be in her 60s. The injured people were from Britain, America, Israel and Australia.
Police arrested a 19-year-old Norwegian national of Somali origin. They say there is no evidence of radicalisation.
Speaking at a press conference, the Met’s assistant commissioner for specialist operations, Mark Rowley, said the police investigation “increasingly points” to the attack being “triggered by mental health issues”.
Mr Rowley had earlier said the force was considering terrorism as “one line of inquiry for us to explore”.
Asked why he had said this, Mr Rowley said it was necessary to “consider all of the possibilities” following recent terror attacks across Europe.
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.”
He added that an address had been searched by officers in north London on Thursday morning and another will be searched in south London.
The man was arrested after a Taser was discharged by an officer.
Scotland Yard said two other women and three men received various injuries in the attack, which happened near the Imperial Hotel.
Two from the group remain in hospital while the other three people have since been discharged, the force added. None have life-threatening injuries, Mr Rowley said.
After receiving treatment in hospital the arrested man, who was detained in Bedford Place, is now in police custody in a south London police station.
Mr Rowley earlier said the force’s homicide command was leading the investigation into the attack, with support from the counter-terrorism unit.
Jodie Parry, who witnessed the events unfold from her hotel window, said she saw a black man running down the street.
“I could hear the policeman screaming ‘stop, don’t move, don’t go any further, just stay where you are’, and he turned round and continued running,” she said.
She added that the man was carrying a knife and had blood on his hands.
Paul Ó Geibheannaigh, who lives near to the square, said he had seen the body of the woman “on the pavement” and the area was surrounded by a “heavy armed police presence”.
“I haven’t seen a police presence like that in this area since 7/7,” he said, referring to the terror attacks on the London transport system in July 2005.
Zuhair Awartani was trying to return to a hotel where he was staying when he saw a “dark-skinned man” in his “late 20s” being arrested.
Mr Rowley said there would be an increased police presence on the streets of London in the wake of the attack – including armed officers – to “provide reassurance and safety”.
Speaking earlier about a motive for the attack, he said officers would keep an “open mind”, and as such terrorism was “one line of inquiry for us to explore”.
The Met’s commissioner Sir Bernard Hogan-Howe said the force’s “swift response undoubtedly helped to prevent more people from getting injured”.
He added: “We will protect our capital and those who live, work and visit our city.”
The Met said the attack was not linked with a separate stabbing in Wandsworth, south-west London, on Wednesday, in which one man was killed and two others injured.
Following the attack, Mr Khan has cut short his holiday and will hold a briefing at City Hall at 15:30 BST.
He said police were doing an “incredibly difficult job” and the safety of Londoners was his “number one priority”.
At the scene
By Emma Atkinson, BBC News
Walking around the park at Russell Square, a busy area for tourists with two hotels, cafes and a few souvenir shops, it almost feels like just another weekday morning. A few dog walkers and commuters buying their first coffee of the day.
But just a few metres away along Southampton Row, a different story has been taking place.
Earlier, police tape cordoned off a wide section of road and a white and yellow tent marked the point where one of the stabbings took place.
Two small bunches of flowers have been left along the park railings.
The forensic tent has now been cleared away and the pavement scrubbed clean, as officers gather the remnants of a few first aid bags that were left scattered on the pavement from the emergency teams.
The area along Southampton Row is now being released from the police cordon and the blue and white tape is being taken down.
Officers in high visibility vests are still gathered, and a few TV cameras remain, but the street is beginning to look like normal again.
Shyam Sanghani, who runs Imperial News and Gifts opposite the scene of the attack, said: “We had more police announced yesterday and then this today, it’s scary.
“We had Russell Square bombing just close by and now this. We’ve been here a long time and to have two incidents is awful. This is our home.”
Emma and Mark Pigdon, from County Durham, who were staying in the President hotel, had been to the theatre when they came back and saw a woman lying on the street.
Emma Pigdon said: “She was a small lady, really slight, which made me think she was a teenager. We were shocked when we later heard she was in her 60s.
“She had what looked like a pattern on her clothes but I think it was blood.”
Kevin Hurley, former lead on counter terrorism at City of London Police, told BBC Radio 4’s Today programme the attack could have happened “any day of the week in any street in our cities, because there are lots of mentally disordered people who should be in hospital and who are out and about”.
“This is not unusual. We should not jump to conclusions and say that this is terrorist-related.”
A police forensics tent was earlier put up on Southampton Row, outside the park in the centre of the square, which is close to the British Museum, but has since been removed.
There had been a heavy police presence in the area.
Earlier on Wednesday, Metropolitan Police commissioner Sir Bernard Hogan-Howe and London Mayor Sadiq Khan had said that there would be more armed police seen on patrol in London.
Sir Bernard announced the move to reassure the public and deter attackers following terror attacks in Europe.
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: