The teenager was finally nabbed outside the Clarendon Hotel after running away down Bedford Place. Police sources confirmed the attacker hailed from Norway and is of Somali descent.
After speaking to his family, searching his home and working with Mi5 and Mi6, police sources added that he has shown no signs of radicalisation.
The dead woman was confirmed as an American tourist while the injured were British, Australian, Israeli and American.
She died at the scene while being cradled by a group of Spanish tourists who rushed to her aid late last night.
The victim, in her 60s, was knifed in the back while five others were injured during the horrific attack.
As she lay dying in the arms of her desperate rescuers, she warned: “He’s still here, he’s still here.”
Hotel guest Chanel Britton, 19, of Barnsley, caught the arrest on her mobile phone.
She told The Sun: “We had just got back and were sat having a drink at the window. Then we saw this man run down the street followed by police with their guns out.
“I went out and they shouted ‘get down’ but he carried on running. They shot him with a taser and he fell to the ground.
“He was a black man in a white t-shirt. They were on the floor for about 40 minutes.
“There was a lot of screaming.”
Witnesses described victims "screaming and covered in blood" following the rampage in which the woman was brutally knifed around 10.30pm last night.
Other terrified onlookers described the "disturbed" knifeman chasing pedestrians and "lunging for anyone he could see" as police confirmed mental health was a likely factor.
A 19-year-old man was arrested at the scene after being tasered by police in Russell Square as officers yelled: "Put it down."
The Norwegian foreign office confirmed the suspect emigrated to the UK from the Scandinavian country in 2002 when he was five.
The horrific incident happened just yards from where one of the 7/7 terror attack bombs detonated in 2005.
London mayor Sadiq Khan urged people to remain "calm and vigilant".
What we know so far about the Russell Square stabbings:
A 19-year-old Norwegian man of Somali descent attacked six people with a knife in central London's Russell Square at around 10.30pm last night Detectives could see no evidence that he had been radicalised An American woman in her 60s was killed and another five injured. Two remain in hospital.
The five injured hail from Britain, Australia, Israel and the USA. The attacker has been arrested on suspicion of murder and is being held at a South London police station.
Police confirmed they were not ruling out the incident being terror-related and said they believed mental health was a key factor.
Fernando, 40, a paralegal from Brazil, said: "I was cycling by and people screamed at me to stop. It was a Spanish family who approached me.
"It was a mother and father and two sisters. Then I saw a white lady, in her sixties, slumped against the railings.
"She had her head in a Spanish woman's lap, the lady, who was trying to keep her alive by talking to her.
"They were not related, I think the Spanish family were just passing by.
"She had been stabbed in the back. She was bleeding. There was an English man who was white, I'd say around 45. He had been stabbed in the side.
"Then there was another lady. I didn't know her nationality but I think she was white. She was in her twenties. She had been stabbed in her arm.
Poster Comment:
He might be crazy but you notice that the black man only attacked white people.