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.
The Met Police's assistant commissioner for specialist operations, Mark Rowley, said the investigation was increasingly pointing to the attack being "triggered by mental health issues".
He had earlier said the force was considering terrorism as a line of inquiry. Mr Rowley, Britain's most senior counter-terrorism officer, told a press conference it had been necessary to consider "all possibilities" following recent terror attacks across Europe.
Armed police were called to the square shortly after 22:30 BST on Wednesday after receiving reports of a man attacking people with a knife.
They arrived within six minutes and chased the suspect, who eyewitnesses said had blood on his hands. They said police ordered him to stand still but he kept running.
He was then Tasered by officers.
Two other women and three men received various injuries in the attack, which happened near the Imperial Hotel.
One person from the group remains in hospital while the others have since been discharged. None of the injuries were life-threatening.
The US ambassador to the UK, Matthew Barzun, said of the woman who died: "Heartbreaking news that a US citizen was killed in #RussellSquare attack. My prayers are with all the victims and their loved ones."
After receiving treatment in hospital, the arrested man, who was detained in Bedford Place, is now in police custody in south London.
Norwegian police said in a statement the man had emigrated from Norway in 2002 and they were assisting London police.
The Met said they had searched an address in north London on Thursday morning and another will be searched in south London.
Jodie Parry, who witnessed the events unfold from her hotel window, said she saw a 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.
'Body on the street'
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".
Mr Rowley said there would be an increased police presence on the streets in the wake of the attack - including armed officers - to "provide reassurance and safety".
Met commissioner Sir Bernard Hogan-Howe said the force's "swift response undoubtedly helped to prevent more people from getting injured".
At the scene
By Emma Atkinson, BBC News
Russell Square is bustling with local workers and tourists exploring the area. It's hard to believe that just a few hours ago it was an active crime scene.
Along Southampton Row, police tape had cordoned off a wide section of road, and a white and yellow forensics tent marked the point where the fatal stabbing took place.
Now the pavement has been scrubbed clean and officers have gathered up the discarded first aid bags left scattered on the roadside by the emergency teams.
While a few TV cameras remain, the street is beginning to look like normal again, with only a small but growing number of floral have been left along the park railings.
Shyam Sanghani, who runs Imperial News and Gifts opposite the scene of the attack, said: "It's scary.
"We had the 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 in the street.
Mrs 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."
London Mayor Sadiq Khan cut short his holiday following the attack 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".
The Met said the attack was not linked to a separate stabbing in Wandsworth, south-west London, on Wednesday, in which one man was killed and two others injured.
Earlier on Wednesday, Sir Bernard and Mr 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.
Knife crime statistics
The Met Police says it was called to more than 9,000 crimes in London in which a knife or sharp instrument was used in the 12 months to April 2016.
The numbers are down from a high of more than 14,000 offences recorded in 2007-08 - although the figures began rising again in the 2014-15 period.
Overall, across England and Wales there has been a 12% decrease in knife crime offences in the last five years.
But, like the trend seen in the capital, the latest data from the Office for National Statistics shows offences were up last year - there were 28,664 incidents across the country in 2015-16, a 10% rise on the previous 12 months.
There were 214 suspected homicides in England and Wales where a knife or sharp instrument was used in 2015-2016 - the total number of deaths has varied between 187 and 237 in the past five years.
Source: Met Police/ONS