A man wielding a knife in a historic London square killed an American woman and wounded five people Wednesday in an attack authorities believed was possibly linked to “mental health” issues.

Police identified the attacker as a 19-year-old Norwegian with Somali origins. The London area’s assistant police commissioner, Mark Rowley, said the investigation “increasingly points to mental health issues,” but officials left open probes into any possible terrorism ties.

“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 names of the slain woman and the wounded victims were not immediately made public. Rowley said those hurt included people of British, American, Israeli and Australian nationalities. None had life-threatening injuries.

Rowley said it appeared to be a “spontaneous attack and that the victims were selected at random” in Russell Square, an area of hotels, pubs and tourist sites including the British Museum.

The suspect was subdued with a Taser shortly after police responded to reports of an attack about 10:30 p.m., according to a police statement.

London’s mayor, Sadiq Khan, said in a statement that police spoke with the man “and are seeking to establish the full facts including motives for this attack. I urge all Londoners to remain calm and vigilant.”

Two of the wounded remained hospitalized early Thursday, while the three others were discharged, police said. Details about the injuries were not released.

The BBC quoted a witness who saw the man being chased and Tasered. He “kept screaming his guts out” and “police were screaming, ‘stop, stop, stop,'” the witness said.

London’s homicide unit was investigating the incident with the support of a counterterrorism unit. The city planned to increase police presence throughout the city Thursday morning “to provide reassurance and safety,” police said.

Attacks with knives and axes have been spreading across Europe, particularly in France and Germany, with responsibility frequently claimed by the Islamic State.

Just hours before the attack, London’s mayor and police announced an increase in the number of armed officers to protect citizens “against the threat of terrorism,” according to the police.

The plan, called Operation Hercules, added 600 more armed guards to monitor the city. The operation was not the result of any specific intelligence, the city said, but a long-term plan to “make London as hostile an environment as possible for any would-be attacker.”

“‎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,” Khan said in a statement Wednesday afternoon.

People near Russell Square posted images of the police barricade to social media. The historic square is popular among tourists and students at the nearby University of London and University College of London in an area that is frequented at all hours of the day and night.