A woman was killed and five people were injured in a knife attack in central London's Russell Square on Wednesday, London Metropolitan Police said.
The police said they were called at 10:33 p.m. (2133 GMT) after a man attacked people with a knife.
London Metropolitan Police Assistant Commissioner Mark Rowley said "mental health is a significant factor in this case and that is one major line of inquiry."
"But of course at this stage we should keep an open mind regarding motive and consequently terrorism as a motivation remains but one line of inquiry for us to explore," Rowley told Reuters.
The police used a Taser electric shock gun and arrested a 19-year-old suspect.
Rowley said counter-terrorism officers are supporting the investigation but they are not leading it. The homicide command was leading the investigation into the attack.
The woman, who was one of the victims, was treated at the scene but was pronounced dead a short time later. Rowley added that the other five victims, including one woman and four men, are being treated.
The incident comes just hours after officials announced the start of Operation Hercules. This is an initiative that included the addition of 600 armed officers across the capital to better protect the city after the recent terror attacks in Europe.
In a recent statement, Mayor Sadiq Khan said: "The threat level here in London has not changed, but it does remain at severe and especially in light of recent deadly attacks in Europe it is important we are prepared should the unthinkable happen."
"We will see more armed officers on our streets, but there is no reason to be alarmed," he added in his statement.