British detectives questioning a 19-year-old Norwegian of Somali origin over a stabbing spree in London said Thursday they had found no evidence of terror motives or radicalisation.

Assistant Commissioner Mark Rowley told reporters the one woman killed in the attack in central London's Russell Square late Wednesday was a US national. Police said earlier that she was thought to be in her 60s.

Among the two women and three men also injured were US, British, Australian and Israeli nationals. Three of the five have been discharged from hospital and the other two are not in a life-threatening condition.

The investigation "increasingly points to this tragic incident as having been triggered by mental health issues," Rowley said outside London's New Scotland Yard police headquarters.

"We believe this was a spontaneous attack and the victims were selected at random.

"We have found no evidence of radicalisation or anything that would suggest the man in our custody was motivated by terrorism."

He said their work so far does not suggest that the suspect's Norwegian nationality and Somali ancestry "are relevant to the motivation for his actions".

He said an address had been searched in north London, while another in south London will be searched.

Detectives have interviewed the suspect and spoken to his family.

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Witnesses have also been interviewed, while a full intelligence review was carried out by the police and the security services.

Rowley called it a "horrific murder and attack".

Scotland Yard is in contact with the relevant embassies, he added.

He said the suspect was shot with an electric shock gun and detained by armed officers within six minutes of the first call to the emergency services.