Mrs Horton's death was confirmed in a statement issued by Florida State University (FSU), where her husband works as a psychology scholar.

She and her husband, named by the university as Richard Wagner, were in London for a summer course and were due to return to the US today.

FSU president John Thrasher said: "There are no words to express our heartache over this terrible tragedy."

The 19-year-old arrested on suspicion of murder following the knife attack, a Norwegian national who is believed to suffer from mental health problems, remains in custody.

Metropolitan Police Assistant Commissioner Mark Rowley said it appeared terrorism was not a motive in the Russell Square attack.

He said: "Whilst the investigation is not yet complete, all of the work we have done so far increasingly points to this tragic incident having been triggered by mental health issues."

Mr Rowley described it as a "spontaneous attack and the victims were selected at random".

'No evidence of radicalisation'

Police were called to reports of an armed man attacking people at 10.33pm on Wednesday.

A suspect was arrested on suspicion of murder six minutes later after being taken out by a Taser.

An unnamed woman in her 60s, now confirmed as an American national, was treated at the scene but was pronounced dead a short time later. 

Police activity at the scene in Russell Square | Photo: PA Images

The other victims were a British man, an American man, an Australian man, an Australian woman, and an Israeli woman.

The Briton remains in hospital, while the four other injured people have been discharged.

Police have interviewed the suspect, spoken to his family, talked to witnesses and searched properties in north and south London, including the 19-year-old's home.

Mr Rowley said "so far we have found no evidence of radicalisation".

As a precautionary measure Mr Rowley said extra police would be deployed in London on Thursday to provide reassurance.

Speaking to Sky News, London Mayor Sadiq Khan praised the quick police response to the incident, adding: "London is no less safe today than it has been in the recent past."

He said: "London is safe. The police have been working their socks off since the first call was made to the police.

"There is no evidence at all that this man was motivated as a consequence of Daesh or similar sorts ... of groups.

"It does appear to be somebody who had mental mental health problems, according to the police."