A woman was killed and five people were hurt when a man randomly went on a murder spree with knife, which took place at Russell Square, near to the British Museum in central London.
The victim, who died in the knife attack was in her sixties and was treated by paramedics after she was hurt by the attacker in the scene but she died after sometime.
Two other women and three men suffered many injuries in the same attack.
The 19 year-old attacker was arrested six minutes later with one of the officers firing a Taser electroshock gun.
According to reports by bbc.com, "Early indications suggest that mental health is a significant factor in this case and that is one major line of inquiry," London police's counter terror chief Mark Rowley said in a statement.
"But of course at this stage we should keep an open mind regarding motive and consequently terrorism as a motivation remains but one line of inquiry for us to explore," added Rowley.
Russell Square is a busy tourist place close to the British Museum and the University of London with a chain of big hotels.
According to the reports by AFP, Xavery Richert, 22, a French tourist staying in youth hostel on the square, told AFP: "I was buying a beer when I heard a woman shouting who was being chased by a man.
"I thought it was a bag snatching... she was not hurt. I came out for a cigarette, I went back, there were firefighters, police, and then I saw the body under a sheet. You could only see the feet sticking out.
Russell Square resident Constantine Somerville said: "It's such a safe area and very quiet especially at night -- why would somebody commit an attack in such a quiet area?"
The Met's assistant commissioner for specialist operations, said the investigation was at an early stage, and that "terrorism as a motivation" was also a line of inquiry for police to explore.
"As a precautionary measure, Londoners will wake up this morning to notice an increased presence on the streets of officers, including armed officers today. We would urge the public to remain calm, alert and vigilant," he said.