THE American woman stabbed to death in London was the wife of a university professor from Florida who was hours away from flying home.
Florida State University said psychology scholar Richard Wagner was teaching summer classes, and his wife had accompanied him to the capital.
The university did not identify his partner, but USA Today named her as Darlene Horton. The couple had planned to return to Tallahassee on Thursday.
Her killer was a man armed with a knife who selected victims at random in a "spontaneous" rampage in the busy Russell Square shortly after 10.30pm on Wednesday.
Ms Horton was killed at the scene and five other people were injured during the attack - the victims suffered a variety of stab wounds including to their chests, back, stomach and arm.
Sky News have named the 19-year-old Norwegian national of Somali origin, who moved to the UK in 2002, as Zakaria Bulhan from south London. He remains in police custody.
Scotland Yard said no evidence has been found of radicalisation or anything to suggest the man in custody was "in any way motivated by terrorism".
Assistant Commissioner Mark Rowley said: "Whilst the investigation is not yet complete, all of the work we have done so far increasingly points to this tragic incident as 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."
Mr Rowley, Britain's most senior counter-terrorism officer, also said armed officers arrived within six minutes of the call, adding: "The man was Tasered and he was detained." No shots were fired.
The episode took place in a popular tourist spot near the British Museum in Bloomsbury, and around a mile to the north of the bustling area around the Strand, where several theatres and scores of restaurants are located.
The events unfolded hours after Scotland Yard announced more armed officers would be deployed on public patrol around the capital after a spate of attacks in mainland Europe.
Met Police Commissioner Sir Bernard Hogan-Howe vowed to protect London, adding: "A normal night-out in our busy capital has ended in horrific circumstances."
London mayor Sadiq Khan called for the public to remain "calm and vigilant".
