Norway's Ministry of Foreign Affairs on Thursday morning confirmed that the man, who is of Somali origin, had left Norway in 2002, when he was just five years old.
"We will assist the British police in the normal manner, with information and any requests they might have in their investigation," a spokesman told the Guardian newspaper.
Norway National Criminal Investigation Service (Kripos) confirmed that the man was a Norwegian citizen at around 1pm on Thursday.
"The NCIS will assist the British police in the normal manner, with information and any requests they might have in their investigation," the service wrote in a press statement. "This is a British investigation and it is the British police who control what they want to be made public in the case and what information they need from the Norwegian police. What we can confirm is that the accused is a Norwegian citizen of 19 years old, registered as emigrated from Norway in 2002."
British detectives questioning the 19-year-old on Thursday said they had as yet found no evidence of terror motives or radicalisation, and that as far as they could tell nothing suggested that the suspect's Norwegian nationality and Somali ancestry were “relevant to the motivation for his actions"
"We have found no evidence of radicalisation or anything that would suggest the man in our custody was motivated by terrorism," Assistant Commissioner Mark Rowley from the London Metropolitan Police told reporters.
He said that the one woman killed in the attack in central London's Russell Square late on Wednesday was a US national. Police said earlier that she was thought to be in her 60s.
Among the two women and three men also injured were US, British, Australian and Israeli nationals. Three of the five have been discharged from hospital and the other two are not in a life-threatening condition.
The investigation "increasingly points to this tragic incident as having been triggered by mental health issues," Rowley said outside London's New Scotland Yard police headquarters. "We believe this was a spontaneous attack and the victims were selected at random."
He said an address had been searched in north London, while another in south London will be searched.
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Detectives have interviewed the suspect and spoken to his family. Witnesses have also been interviewed, while a full intelligence review was carried out by the police and the security services.
Rowley called it a "horrific murder and attack".
Scotland Yard is in contact with the relevant embassies, he added.
He said the suspect was shot with an electric shock gun and detained by armed officers within six minutes of the first call to the emergency services.