London (CNN) The wife of an American professor was killed and five others injured in a knife attack in central London's Russell Square in what police Thursday called a "spontaneous" assault.

London Metropolitan police identified her as Darlene Horton, 64.

She was married to a Florida State University professor, the university said. Richard Wagner, a professor of psychology, and Horton were in London, where he taught a summer session for the university. The couple had planned to return to Tallahassee on Thursday.

The stabbings late Wednesday night rattled the city popular with tourists and came just hours after authorities announced an increased police presence there following recent terror attacks across Europe.

Police said they had arrested a 19-year-old man on suspicion of murder. The suspect is a Norwegian national of Somali origin, and "so far we have found no evidence of radicalization," London Metropolitan Police Assistant Commissioner Mark Rowley said Thursday.

Norway's National Criminal Investigation Service said the suspect, who has not been named, had emigrated from Norway in 2002. It was unclear if he had immigrated to the UK.

Rowley said that authorities' main line of inquiry was focusing on mental health issues, essentially ruling out terrorism as a likely motive.

Journalists and others gather as police guard the attack scene Thursday in London's Russell Square.

Officials briefed on the investigation had earlier told CNN they believed the case was a terror attack based on the initial evidence.

JUST WATCHED Police: 19-year-old arrested in London knife attack Replay More Videos ... MUST WATCH Police: 19-year-old arrested in London knife attack 02:39

"Whilst the investigation is not yet complete -- all of the work that we have done so far, increasingly points to this tragic incident as having been triggered by mental health issues. At this time we believe this was a spontaneous attack and the victims were selected at random," Rowley told journalists.

Police said two other women and three men were injured. The victims were from Britain, Israel, Australia and the United States.

An earlier police statement said two remained hospitalized.

Dying in family's arms

Police were called after 10:30 p.m. Wednesday following reports of a man "seen in possession of a knife injuring people," authorities said in a statement.

People leave #flowers outside the gates of #russellsquare in central #London where an American woman was stabbed. This area is a particularly busy with tourists as there are many hotels here. It is also an area for many international university students and moments away from the British National Museum. A photo posted by 33ely33 (@33ely33) on Aug 4, 2016 at 7:01am PDT

Officers used a stun gun on the suspect, and he was arrested less than 10 minutes later, authorities said.

He was treated in the hospital and is in police custody at a south London station, police said.

A 40-year-old Brazilian man living in London told CNN a Spanish family stopped and asked him to call an ambulance on Wednesday night.

"I thought someone had collapsed or passed out, that was my first impression," said Fernando, who did not want to give his last name.

He said one of those family members was holding the dying woman and helping and reassuring her. She had been stabbed in the back, Fernando said, adding that he was in shock.

"It was not nice to see someone dying," he said.

Philippa Baglee told CNN she saw the aftermath of the attack.

"I came out of the Atrium (bar) to have a cigarette, and I saw people standing over there looking at the ground. I could see someone laying down and a guy with a motorbike helmet balanced on his head just walking up and down," she said.

Janet Pavely, who was with Baglee, said people had to be "very self-aware" when out in public now.

"It could happen anywhere. I don't think it would stop us from coming back. We were lucky last night. We were going to walk, we took a cab instead. Who knows?" she said.

A woman lays flowers at the scene of the knife attack in London's Russell Square on Thursday.

Police were lifting cordons around the crime scene by late Thursday morning. Workers sprayed and scrubbed the sidewalks clean as people began to leave flowers at the site.

Police presence beefed up

Londoners woke up Thursday to a heavier police presence on their streets after the attack in the heart of the city.

Rowley said it was "to provide reassurance and safety. We ask the public to remain calm, vigilant and alert."

A police officer guards the scene of the knife attack Thursday at Russell Square in London.

Mayor Sadiq Khan told CNN that he wanted to reassure people across the globe "that London is one of the safest capitals in the world."

"It is a reality in 2016 -- especially when you look at what happened in Nice , in Brussels , in Munich , in parts of America -- we've always got to be vigilant and never complacent," he said.

Khan said it was reassuring that the police investigation found the suspect was not an ISIS-inspired extremist, but someone who appeared to have mental health issues.

Armed police response units were at the scene within six minutes of the attack, he told CNN.

One Twitter user identifying herself as a resident in the Russell Square area thanked the London police for their quick response.

Thank you so much @metpoliceuk for being so fast, caring and efficient tonight in #russellsquare . I am a resident and I really thank you. — susanna cappellaro (@SuzieCappellaro) August 4, 2016

A city rattled

The attack, however, left both Londoners and foreign visitors shocked.

JUST WATCHED London's armed cops readying for terror attack Replay More Videos ... MUST WATCH London's armed cops readying for terror attack 01:09

Mohammed Jithin, 28, an Indian tourist visiting with his family, said he had walked past Russell Square on the day of the attack.

"I studied in London, so I used to come here frequently. It makes me fear because I have my family with me -- I'm not the student anymore. ... There is an insecurity feeling now throughout our visit," he told CNN Thursday outside the British Museum near the crime scene.

An ambulance crew removes a body from Russell Square early Thursday after a woman was killed.

Julia Fournier, a 20-year-old student from France, said she was rattled by the attack, which happened close to the dormitory where she is staying. But she said she was not really scared to go out.

"This is something that happens all the time -- I'm French. If we have to worry about things like that all the time, we can't live in peace with ourselves. We can't feel insecure all the time, but I do feel safer in other parts of the world. I'm used to the city," she said.

Menna Rawlings, British high commissioner to Australia, said on Twitter she was "shocked to hear of #RussellSquare stabbings; and that Australians are among those injured."

Shocked to hear of #RussellSquare stabbings; and that Australians are among those injured. Thoughts with all affected. Proud of our police. — Menna Rawlings (@MennaRawlings) August 4, 2016

Russell Square

A woman in her 60s has died and 5 others are injured in a knife attack in #London's #russellsquare Police say there are keeping an "open mind regarding motive, terrorism remains one line of inquiry being explored." A photo posted by Isa Soares (@isasoarescnn) on Aug 3, 2016 at 9:54pm PDT

Russell Square, in the Bloomsbury district of west-central London, is a busy passing point in the daytime to some of the city's attractions, including the British Museum and the main buildings of the University of London.

But it isn't particularly bustling at night compared with other nearby neighborhoods that are home to more restaurants and bars.

In the July 7, 2005, terror attacks on London, the most devastating of the four bombs hit the Underground subway line running close to Russell Square.