An official with London's Metropolitan Police said there was no evidence of terrorism in a deadly stabbing on Wednesday night. One American woman was killed and five other people were injured in the attack.  LONDON — A 19-year-old Norwegian man was arrested on suspicion of murder after a knife attack in Central London that killed an American woman and wounded five people, the Metropolitan Police said on Thursday.  The attack on Wednesday night, on the eastern corner of Russell Square in Bloomsbury — a neighborhood known for its handsome garden squares that is home to the British Museum and several universities — immediately raised fears about terrorism. But after investigating through the night, the police said there was no evidence that the attack was politically motivated.  “Whilst the investigation is not yet complete, all of the work that we have done so far increasingly points to this tragic incident as having been triggered by mental health issues,” Assistant Commissioner Mark Rowley, the top counterterrorism official at the Metropolitan Police, said in a statement. “At this time we believe this was a spontaneous attack and the victims were selected at random.”  The police had said in a statement early Thursday morning that terrorism was “one line of inquiry being explored,” but they later backed away from that theory.  “So far we have found no evidence of radicalization or anything that would suggest the man in our custody was motivated by terrorism,” Mr. Rowley told reporters outside New Scotland Yard, the Police Headquarters.  Mr. Rowley said the attacker was a Norwegian citizen of Somali ancestry, but that his background did not appear to be “relevant to the motivation for his actions.”  The woman who was killed was an American citizen, and the five wounded people, two women and three men, were citizens of Australia, Britain, Israel and the United States, Mr. Rowley said. All were hospitalized, and three were treated and released.  Scotland Yard confirmed late Thursday that the American woman who was stabbed to death was Darlene Horton, 64, the wife of an American professor of psychology from Florida State University, Richard Wagner. Professor Wagner had been teaching in a summer program in London.  The summer program had ended, and the couple had planned to fly home on Thursday, according to a statement released by the university.  “There are no words to express our heartache over this terrible tragedy,” said John Thrasher, Florida State’s president. “We will do all we can to assist Professor Wagner and his loved ones, as well as his friends and colleagues in the psychology department, as they mourn.”  In Norway, the National Criminal Investigation Service said in a statement that it was cooperating with the British authorities. The man immigrated to Britain from Norway as a child in 2002, the agency said.  The first call about the attack came at 10:33 p.m., the London police said.  Armed officers arrived six minutes later and used a stun gun to disable the suspect. He was treated at a hospital and then taken into custody at a police station in South London. “This morning, we have searched an address in North London and will search another in South London,” Mr. Rowley told reporters.  Helen Edwards, 33, who lives in the area, was passing through Russell Square when she came across armed police officers, a rare sight in London.  “When I arrived, the police cordon was up, there were a lot of armed police, police cars, ambulance,” she said in a telephone interview, estimating that she had arrived about an hour after the attack. She began taking pictures and stayed in the area off and on, into the morning.  “It was only yesterday that the police were announcing having more armed police on the streets of London,” Ms. Edwards said. “It was almost like because that was in my mind, it wasn’t quite as much of a surprise as it perhaps would have been otherwise.”  “There wasn’t a huge sense of panic; I couldn’t tell whether it was some kind of incident or whether it was just an alert,” she said. “It was obvious that if something had happened it was under control.”  Mayor Sadiq Khan said in a statement on Thursday that the “safety of all Londoners” was his top priority and expressed his sympathy for the victims.  The attack occurred just hours after officials announced the start of Operation Hercules, an initiative that included the addition of 600 armed officers in the city to better protect it against an attack.  Handguns were effectively banned in Britain after a school massacre in 1996, and most police officers do not carry firearms. Mr. Rowley said of the police who responded: “They detained an armed and dangerous man, and resolved it using the minimum necessary force: No shots were fired. We should be proud of them and the British tradition of using the minimum necessary force.”  Mr. Khan, who was elected in May, vowed during his campaign to “challenge gang culture and knife crime head on,” and recently said, after a fatal stabbing in North London, that he was “extremely worried” about knife crime.  The police have been on heightened alert after a series of terrorist attacks in Europe.  “We have all watched the recent terrorist atrocities unfold with a terrifying and depressing sense of horror and dread,” Bernard Hogan-Howe, the Metropolitan Police commissioner, wrote in a commentary last weekend in the newspaper The Mail on Sunday, in which he announced the deployment of the 600 additional armed officers.  On July 7, 2005, 52 people were killed in four suicide bombings in London — two of which were not far from Russell Square — set off by Islamist militants.  The most recent murder in the country linked to terrorism occurred on May 22, 2013, when an off-duty Army soldier, Lee Rigby, was run over with a car and then hacked to death near a barracks in Woolwich, in Southeast London.  Reflecting on the heightened state of security across Europe, Ms. Edwards, the resident who came across the aftermath of the crime, said: “I don’t feel unsafe. I think every major city comes with its risks.”  She added: “London as a city has been through a lot, has survived a lot and the people survived a lot. It’s at the back of your mind. But I think that if you’re constantly worried about it, you just wouldn’t live here anyway.”  Follow Dan Bilefsky @DanBilefsky and Palko Karasz @karaszpalko on Twitter.  Reporting was contributed by Sewell Chan from London, Henrik Pryser Libell from Oslo, and Niraj Chokshi, Christopher Mele and Richard Pérez-Peña from New York.
The police said “mental health issues” appeared to have led to the attack in Russell Square, which also left five people wounded.