A US citizen in her 60s was killed in Wednesday night’s knife attack in Russell Square, Bloomsbury, in which five other people were injured, the London Assistant Police Commissioner has confirmed.

The woman was identified as a 64-year-old Darlene Horton, police said adding that her postmortem examination will take place on Friday.

Authorities also said that “specially trained police liaison officers” have been dispatched to speak with her family.

London Assistant Police Commissioner, Mark Rowley said among the injured were Australian, British and Israeli citizens. None have life-threatening conditions.

He said the attacker was a Norwegian man of Somali origin.

AC Rowley: The suspect is a Norwegian national of Somali ancestry. No evidence of radicalisation or that the man was motivated by terrorism. — Metropolitan Police (@metpoliceuk) August 4, 2016

Police investigating the attacks are close to ruling out a terror motive, saying “mental health” was a significant factor.

"The investigation is fast paced and we are making urgent progress.

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"While the investigation is not yet complete, all of the work that we have done so far increasingly points to having been triggered by mental health issues.

"At this time we believe this was a spontaneous attack and that the victims were selected at random."

The 19-year-old suspect has been arrested on suspicion of murder and is being detained in a South London police station.

Police say they have interviewed the suspect’s family and searched his address. They have also liaised with MI5 and MI6, and neither was aware of him.

Rowley said police had "searched addresses in north London" and would search another in south London.

"I emphasise that so far we have found no evidence of radicalisation that would suggest that the man in our custody is in any way motivated by terrorism."

Rowley said the police started to receive calls from the public at 10.33pm local time to reports of a man attacking people with a knife.

Armed officers were there within six minutes, when they arrested the suspect after one of the officers took him down using a Taser, he said.

"Our firearms officers bravely rushed to that scene with very little information.

"They would not have known if this was an act of terrorism or a random attack we now know it to be.

"They detained an armed and dangerous man - and resolved it using the minimum necessary force - no shots were fired.

"We should be proud of them and the British tradition of using the minimum necessary force."

Rowley said as a precautionary measure, the number of officers on the streets on Thursday have been increased.

Two people injured in the knife attack remain in hospital, while three others have been discharged.