LONDON (AP) -- Police put more officers on London streets Thursday after a teenager stabbed a woman to death and injured five other people near the British Museum. Mental health may have been a factor in the attack, police said, but "terrorism remains one line of inquiry."
The incident came just days after authorities had warned the public to be vigilant in light of attacks inspired by the Islamic State group elsewhere in Europe.
Assistant Commissioner Mark Rowley, head of counterterrorism at the Metropolitan Police, said early indications suggested that mental health was a factor in the attack, which occurred at around 10:30 p.m. (2130GMT, 5:30 p.m. EDT) Wednesday in Russell Square, a busy central area full of students and tourists.
Rowley said police "are keeping an open mind regarding the motive, and terrorism remains one line of inquiry being explored."
Metropolitan Police Commissioner Bernard Hogan-Howe said "mental health remains a substantial focus for our investigation."
Police said they received "numerous" calls from members of the public about a man attacking people with a knife in the streets around the square.
Officers used a stun gun to subdue a 19-year-old suspect, who was arrested on suspicion of murder.
Police said the woman who died was in her 60s but gave no other details. Two victims remain in the hospital, while others have been discharged.
Helen Edwards, 33, who lives in the area, came out for a walk late Wednesday and found armed police near a subway station. In a city with vivid memories of the July 7, 2005, attacks on public transport — two of which struck near Russell Square — she immediately suspected that an attack had occurred.
"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."
Ellie Cattle, 21, a student staying in a hotel near the square, said she heard police shouting: "'Put it down, put it down!'
"Then I heard what sounded like a gunshot, but it must have been the Taser," she said. "After that they just stopped shouting. I didn't hear any screams from anyone."
London Mayor Sadiq Khan urged the public to keep calm and remain vigilant, and encouraged people to be the first line of defense 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.
Knives are the most common murder weapon in Britain, which has strict gun-control laws. There were 186 knife killings in the year to March 2015, according to government statistics — a third of all murders.
In the last three years London has seen two knife attacks by people inspired by radical Islam. In May 2013, two al-Qaida-inspired London men killed off-duty soldier Lee Rigby in the street near his barracks. In January, mentally ill Muhiddin Mire tried to behead a London Underground passenger, shouting that he was doing it "for Syria."
The Russell Square incident 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 Islamic State-inspired groups in Europe.
Police in Britain do not carry guns for the most part — a principle that remains unchanged. Even with the additional armed officers, most of London's 31,000 police officers will not be armed.