"We're investigating this as an attempt at carrying out an act of terrorism", Assistant Chief of Police Rune Skjold told a news conference on Sunday.
Earlier this weekend, investigators found the body of a woman related to the suspected assailant. Skjold called her death suspicious and said that the police are searching for more information on the matter. The police later said that the woman was the shooter's stepsister.
The shooter, detained by the police, has been charged with murder.
At least a dozen people were in the Al-Noor Centre when an attacker started firing. He was quickly overpowered by the people there, and only managed to wound a 75-year-old man.
The attack on the mosque occurred as Muslims across the world celebrated the second day of the annual Hajj pilgrimage to Mecca, Saudi Arabia.
