Terrorism is one potential motive for a knife attack in which one woman was killed and five others injured, police say.
Police were called to reports of an armed man attacking pedestrians in Russell Square at 10.33 pm on Wednesday.
Six minutes later, a 19-year-old was hit with a Taser and arrested.
He is in police custody held on suspicion of murder. Police say he is a Norwegian national of Somali origin, but no evidence has been found of radicalisation.
Police said a US citizen was killed in the attack. Two of the injured victims remain in hospital. Three others have been discharged.
An unnamed woman, believed to be in her 60s, was treated at the scene but pronounced dead a short time later. Five others – a woman and four men – were injured.
The most senior counter-terrorism officer, Commander Mark Rowley, said that mental health may be a ‘significant factor’ but terrorism was ‘one line of inquiry that we should explore’.
The attack came on the same day that the Met announced it would increase the number of armed patrols in London as an anti-terrorism measure.