BREAKING: 1 dead, up to 6 injured in London knife attack, police say. https://t.co/0Mwzn99JHJ— ABC News (@ABC) August 4, 2016
The London force sent extra officers into the city streets and mobilized counterterror detectives, but said the investigation suggested mental illness was the driving force behind the attack.
The attack came just days after authorities warned the British public to be vigilant in light of attacks inspired by the Islamic State group elsewhere in Europe.
Metropolitan Police Assistant Commissioner Mark Rowley said the investigation "increasingly points to this tragic incident as having been triggered by mental-health issues."
"So far we have found no evidence of radicalization or anything that would suggest the man in our custody was motivated by terrorism," Rowley said.
The name of the dead woman, thought to be in her 60s, has not been released. Rowley said the five injured people are British, American, Israeli and Australian, and none has life-threatening injuries. Two remain in a hospital, while three others have been discharged.
Rowley said it appeared to be a "spontaneous attack and that the victims were selected at random."
Officers used a stun gun to subdue the 19-year-old suspect, who was arrested on suspicion of murder. Detectives from the force's murder and terrorism squads interviewed the suspect, his family and witnesses and searched properties, and found no evidence of radicalization, Rowley said.
The National Criminal Investigation Service in Norway confirmed the suspect was a 19-year-old Norwegian national who had left the country in 2002.
Police said they received "numerous" calls from members of the public at around 10:30 p.m. (2130GMT, 5:30 p.m. EDT) Wednesday about a man attacking people with a knife in the streets around Russell Square, a busy central area full of students and tourists.
Student Megan Sharrock, 18, looked out her window and saw someone lying on the sidewalk under a blanket.
"There was like two rivers of blood running away from the person so we thought, yeah, someone has been killed," she said.
Helen Edwards, 33, who lives in the area, came out for a walk late Wednesday and found armed police near a subway station. In a city with vivid memories of the July 7, 2005, attacks on public transport - two of which struck near Russell Square - she immediately suspected that an attack had occurred.
"There is always that thing in the back of your mind," she said. "You live with that threat of terrorism or other crimes in the back of your mind. It wasn't a huge shock I guess."
Ellie Cattle, 21, a student staying in a hotel near the square, said she heard police shouting: "'Put it down, put it down!'
"Then I heard what sounded like a gunshot, but it must have been the Taser," she said. "After that they just stopped shouting. I didn't hear any screams from anyone."
London Mayor Sadiq Khan urged the public to keep calm and remain 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," Khan said.
Knives are the most common murder weapon in Britain, which has strict gun-control laws. There were 186 knife killings in the year to March 2015, according to government statistics - a third of all murders.
In the last three years London has seen two knife attacks by people inspired by radical Islam. In May 2013, two al-Qaida-inspired London men killed off-duty soldier Lee Rigby in the street near his barracks. In January, mentally ill Muhiddin Mire tried to behead a London Underground passenger, shouting that he was doing it "for Syria."
The Russell Square incident came within hours of an announcement by London police that they were putting more armed officers on the streets. The idea was to sustain public confidence following attacks by Islamic State-inspired groups in Europe.
Police in Britain do not carry guns for the most part - a principle that remains unchanged. Even with the additional armed officers, most of London's 31,000 police officers will not be armed.