The Danish citizen arrested over a deadly bow and arrow attack in Norway had converted to Islam and there were fears he had been radicalised, police have said.
The 37-year-old is accused of killing four women and a man on Wednesday night in the southern town of Kongsberg.
Regional police chief Ole Bredrup Saeverud said officers had last been in touch with the man in 2020.
He was arrested and questioned for several hours overnight.
The victims were all aged between 50 and 70, Chief Saeverud told reporters on Thursday morning