Police had concerns about signs of radicalisation in the Danish convert to Islam who is suspected of killing five people with a bow-and-arrow and other weapons in a Norwegian town, a senior officer said on Thursday.

Flags flew at half mast across Kongsberg where the victims, four women and one man aged between 50 and 70, were killed. Two people, including an off-duty police officer, were wounded in the attack on Wednesday evening.

The 37-year-old suspect was in custody and was believed to have acted alone, police said. His lawyer said he was cooperating with police.

"The events at Kongsberg appear at the moment to be an act of terror," the PST security police said in a statement, adding the investigation will determine the motive.

The suspect, who has not been identified by police, had converted to Islam and police had been worried about signs of his radicalisation, regional police chief Ole Bredrup Saeverud told a news conference.