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 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.

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. - Mark Rowley, Metropolitan Police assistant commissioner

"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 name of the dead woman, thought to be in her 60s, hasn't been released. U.S. Ambassador Matthew Barzun confirmed she was American, tweeting: "Heartbreaking news that a U.S. citizen was killed in #RussellSquare attack. My prayers are with all the victims and their loved ones."

Heartbreaking news that a U.S. citizen was killed in <a href="https://twitter.com/hashtag/RussellSquare?src=hash">#RussellSquare</a> attack. My prayers are with all the victims and their loved ones. —@MatthewBarzun

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 remains hospitalized. 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. It came just days after authorities warned the British public to be vigilant in light of attacks inspired by the Islamic State group elsewhere in Europe.

Student Megan Sharrock, 18, looked out her window and saw someone lying on the sidewalk under a blanket.

"There was like two rivers of blood running away from the person so we thought, yeah, someone has been killed," she said.

"It's really shocking, [a] scary world we live in to think that could happen," she said. "That could happen to anyone, just walking down the street."

(Reuters)

Helen Edwards, 33, who lives in the area, came out for a walk and found armed police near a subway station. In a city where memories of the July 7, 2005, attacks on public transport remain raw, she immediately suspected there had been a militant attack.

"There is always that thing in the back of your mind," she said. "You live with that threat of terrorism or other crimes in the back of your mind. It wasn't a huge shock, I guess."

London Mayor Sadiq Khan earlier urged the public to keep calm and remain vigilant, urging people to be the first line of defence against any form of attack.

"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," he said.

Armed patrols

inspired groups in Europe. An armed police officer patrols the scene of Wednesday's deadly knife attack in Russell Square in London. (Neil Hall/Reuters) The Russell Square attack came within hours of an announcement by London police that they were putting more armed officers on the streets. The idea was to sustain public confidence following attacks by ISIS-

Most British police don't carry guns, a principle that remains unchanged. Even with the additional armed officers, the vast majority of London's 31,000 police officers won't be armed.

Armed officers responded to Wednesday's stabbings, but didn't fire any shots.

Rowley said "we should be proud of them and the British tradition of using the minimum necessary force."