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.