Mr Aas said that no other people are currently being searched for in relation to the attack. The wounded have been taken to hospital, but police have not yet released information on their condition.
The attack is one of the worst mass killings in Norway since the 2011 shooting carried out by the domestic terrorist Anders Behring Breivik.
Breivik, a white supremist, killed 77 people in a pair of attacks in both Oslo and on Utoya island. He detonated a car bomb in Oslo next to a tower block housing the former Minister Jens Stoltenberg.
As confused authorities attempted to deal with the bomb, Breivik drove to Utoya island where a group from the youth division of the left wing Labour Party had gathered for a camp. He shot dead 67 people on the island and injured a further 32.
Last month police in Norway reported dozens of disturbances and violent clashes including mass brawls in the country’s cities after streets, bars, restaurants and nightclubs were filled with people celebrating the end of pandemic restrictions.
There were rowdy celebrations by hundreds of citizens across Norway and unrest was reported in several places, including in the southern city of Bergen and the central city of Trondheim, but the situation was worst in Oslo. Police said they registered at least 50 fights and disturbances on a single night in the capital.
Norway has one of the world’s lowest crime rates. Last year Norwegian police, who are usually unarmed, used or threatened to use weapons only 28 times, according to the Justice Ministry.
Earlier this year Professor Nils Christie, a criminology expert at the University of Oslo, said: “Norway is at the bottom of the list of violent crimes per capita. Our biggest crime problem is the unfounded anxiety people feel about it.”