An American woman has been killed and five people wounded by a man armed with a knife in London's Russell Square.
Police said the 19-year-old suspect was a Norwegian of Somali origin, but they have found no signs of radicalization. He was subdued by police using a Taser stun gun, London police said.
"Early indications suggest that mental health was a factor in this horrific attack," Assistant Commissioner Mark Rowley said in a statement.
"However we are keeping an open mind regarding the motive and terrorism remains one line of inquiry being explored."
Wednesday night's attack was less than a mile from the British Museum.
The name of the victim, believed to be in her 60s, was not released. Two other women and three men — British, American, Israeli and Australian — sustained non-life-threatening injuries, police said. Three were later discharged from the hospital, according to police.
Londoners could expect to wake up to an increased police presence, including armed officers, in the city, Rowley said.
London Mayor Sadiq Khan said in a statement Thursday morning that the safety of Londoners was his top priority.
"I urge all Londoners to remain calm and vigilant. Please report anything suspicious to the police. 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."
In an earlier report on its website, the Met said that officers were alerted at 10.33 p.m. BST on Wednesday to a man in possession of a knife who was injuring people at the city-center location.
"Officers attended the scene along with the London Ambulance Service," the Met report said. "Up to six people were found injured at the location. A female (no further details) was treated at the scene but was pronounced dead a short time later."
The report said that a man was arrested at 10.39 p.m. BST and "a Taser was discharged by one of the arresting officers," and that terrorism was a possibility that was being explored.