LONDON — A Somali-Norwegian teenager went on a knife rampage through London’s Russell Square, a hub for students and tourists, fatally stabbing an American woman from Florida and wounding five other people.

Police said Thursday that it wasn’t terrorism – but in a city on edge after a summer of attacks elsewhere in Europe, both authorities and London residents initially responded as if it were. Police flooded the streets with extra officers and mobilized counterterror detectives before saying the shocking burst of violence appeared to have been “triggered by mental-health issues.”

Police officers used a stun gun to subdue the 19-year-old suspect at the scene of the stabbings late Wednesday, among busy streets lined with hotels close to the British Museum.

“Terror in London” ran the headline in the Mail Online, one of several media outlets to speculate that the attack was an act of terrorism. Police initially said terrorism was “one line of inquiry being explored.”

But hours later Metropolitan Police Assistant Commissioner Mark Rowley said “we have found no evidence of radicalization or anything that would suggest the man in our custody was in any way motivated by terrorism.”

He said detectives from the force’s murder and terrorism squads had interviewed the suspect, his family and witnesses and searched properties.

“We believe this was a spontaneous attack and the victims were selected at random,” Rowley said.

London Mayor Sadiq Khan said “there is no evidence at all that this man was motivated by Daesh” – another name for the Islamic State group – or similar organizations.

Rowley said the suspect, whose name hasn’t been released, is a Norwegian of Somali ancestry – though police don’t consider that “relevant to the motivation for his actions.” Norway’s National Criminal Investigation Service said he had left the Scandinavian country in 2002, when he was a small child.

The Metropolitan Police identified the dead woman as 64-year-old Darlene Horton.

Florida State University in Tallahassee said Horton was married to psychology professor Richard Wagner, who had been teaching summer classes in London.

“There are no words to express our heartache over this terrible tragedy,” Florida State University president John Thrasher said in a statement. “We are shocked that such senseless violence has touched our own FSU family.”

Two Australians, an Israeli, an American and a British citizen were wounded, none with life-threatening injuries.

Police said a British man with a stab wound to his stomach was hospitalized in serious but stable condition. The four others were treated and released Thursday.

While knife crime is a regular occurrence in London – there have been two other blade killings this week – the scale and randomness of the rampage rattled nerves.