Oslo police said in a statement that they'd received messages warning them about a man with a bow and arrow in 2020.
"There earlier had been worries of the man having been radicalised,'' Police chief Ole B. Saeverud said.
According to the police chief, the perpetrator in the deadly archery attack in Kongsberg had converted to Islam.
"The offender has converted to Islam," Police chief Ole B. Saeverud added.
He added that the police are asking not to publish photos and videos of the tragedy on social networks.
"After the investigation, it will be known for sure, but the police believe that there are no links between the perpetrator and the victims," he said.
Earlier, he said that all the victims are between 50 and 70 years old. The detained 37-year-old Dane was interrogated at night. According to the police, he admitted that he had done what he was accused of, expressed his willingness to cooperate with the police, but did not explain the motives for the massacre.
On Wednesday night, police detained a man in central Kongsberg who had launched arrows from a bow, killing at least five people and injuring two others. The police have not ruled out that it was terrorist attack. The detainee was a 37-year-old Dane living in Kongsberg, who has been charged.