The woman fatally stabbed in an attack in central London this week has been named by police as Darlene Horton.
The 64-year-old from Florida was pronounced dead at the scene following an attack in Russell Square on Wednesday, which left two other women and three men injured.
A 19-year-old Norwegian national of Somali descent was arrested on suspicion of murder.
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Horton was in London with her husband, Richard Wagner, a psychology professor at Florida State University, reports the Wall Street Journal.
Friends have paid tribute to the "inspirational" retired special-education teacher, who was just hours from returning home to the small Florida city of Tallahassee when she was killed, according to a Press Association report in The Guardian.
The couple, who have two grown-up children, are said to have played an important role in the university's psychology faculty and were highly regarded within their local community for their philanthropy.
Matthew Barzun, the US ambassador to the UK, said the news was "heartbreaking".
The Metropolitan Police released a statement confirming that the suspect, who was armed with a knife, was Tasered and detained and that no shots were fired during the incident. The man was treated in hospital shortly after the attack and is now in custody.
Amateur footage obtained by the BBC shows the man, who had emigrated from Norway in 2002, being restrained by police.
Early reports that a terrorist motive could not be ruled out were later rejected by police, who said there was no evidence the suspect had been radicalised. Investigators believe the attack was spontaneous, with victims "selected at random" as a result of mental health problems.
Officers yesterday searched an address in north London and will search another in south London today.
London stabbing: Norwegian held on suspicion of murder
04 Sugust
A woman was killed and five others injured in a mass stabbing in central London last night. A 19-year-old Norwegian national of Somalian descent has been arrested in connection with the attack.
After earlier saying a terrorist motive could not be ruled out, the Metropolitan Police now say there is no evidence the suspect had been radicalised and believe the attack was a result of mental health problems.
"At this point we believe this is a spontaneous attack and the victims were selected at random," said Assistant Commissioner Mark Rowley.
Given the recent attacks across Europe, Scotland Yard's SO15 counter-terrorism command had initially been called in to assist with the investigation, he added, and a heightened armed police presence would continue across the capital "for as long as necessary".
Police were called to Russell Square at 10.33pm yesterday to find six people had been injured. One woman, a US national in her 60s, was treated at the scene but later pronounced dead.
The injured include British, American, Australian and Israeli citizens. None are thought to have life-threatening injuries.
London Mayor Sadiq Khan, who cut short his holiday to deal with the aftermath of the attack, said his "heart goes out the victims and their loved ones".
He added: "I urge all Londoners to remain calm and vigilant. Please report anything suspicious to the police."
A witness calling himself Paul told BBC Radio London: "There were armed police everywhere, a massive police presence, and a corpse lying in the street covered in a blanket."
Another witness, Jodie Parry, told the BBC she saw a man carrying a knife and with blood on his hands running away from the scene. "I could hear the policeman screaming, 'Stop, don't move, don't go any further, just stay where you are' and he turned around and continued running," she said. The suspect was Tasered by police.
Hundreds of extra armed police have been placed on the streets of London to protect the capital from a possible terrorist attack.
However, local resident Paul O Geibheannaigh, who witnessed the aftermath of last night's stabbing, said he did not think security forces could prevent such incidents.
"If someone comes at you in the street with a knife there's nothing armed police can do about it," he said. "You're in the wrong place at the wrong time."
