US woman killed in 'spontaneous' London stabbing spree

The woman killed in a London stabbing spree on Wednesday was a US national, authorities confirmed hours after identifying the 19-year-old arrested was a Norwegian of Somali orgins.

Among the two women and three men also injured were US, British, Australian and Israeli nationals, according to British authorities who ruled out a terrorist motive for the attack.

The investigation "increasingly points to this tragic incident as having been triggered by mental health issues," Assistant Commissioner Mark Rowley told reporters on Thursday.

"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."

The 19-year-old was detained by police on suspicion of murder soon after the frenzied attack in Russell Square late on Wednesday evening.

Early indications suggested that "mental health was a factor in this horrific attack, however we are keeping an open mind regarding the motive," London Metropolitan Police Assistant Commissioner Mark Rowley said in a statement.

A Taser electric shock gun was used on the knife-wielding suspect.

Rowley said there would be "an increased presence on the streets" on Thursday, including the unusual sight of armed officers in the capital.

The victim, believed to be in her 60 was pronounced dead at the scene after paramedics failed to save her life.

A French tourist who was staying in a nearby hostel said he heard a “woman shouting” while being “chased by a man”.  

On Thursday, London mayor Sadiq Khan urged for calm and vigilance as police sought "to establish the full facts including motives"

"I urge all Londoners to remain calm and vigilant... We all have a vital role to play as eyes and ears for our police and security services and in helping to ensure London is protected."

Earlier this week, Metropolitan Police chief Bernard Hogan-Howe said an attack in London was a question of "when, not if" following a string of deadly assaults on European cities.

However, Britain’s threat level has been “severe” since August 2014 - the second highest level that suggests an attack is "highly likely".

Agencies contributed to this report.