Philip Manshaus, 21, was remanded in custody Monday, suspected of murder and a "terrorist act" that police say he filmed himself committing.
Answering police questions on Friday, "the suspect admits the facts but has not taken a formal position as to the charges," Oslo police official Pal-Fredrik Hjort Kraby said in a statement.
Manshaus is suspected of murdering his 17-year-old step sister Johanne Zhangjia Ihle-Hansen, before entering the Al-Noor mosque in an affluent Oslo suburb and opening fire before he was overpowered by a 65-year-old man.
Just three worshippers were in the mosque at the time, and there were no serious injuries.
Manshaus appeared in court this week with two black eyes and scrapes and bruises to his face, neck and hands.
Police have said he has "extreme right views" and "xenophobic positions" and that he had filmed the mosque attack with a camera mounted on a helmet. He had initially denied the accusations.
The incident came amid a rise in white supremacy attacks around the world, including the recent El Paso massacre in the United States.
Norway witnessed one of the worst-ever attacks by a rightwing extremist in July 2011, when Anders Behring Breivik, who said he feared a "Muslim invasion", killed 77 people in a truck bomb blast near government offices in Oslo and a shooting spree at a Labour Party youth camp on the island of Utøya.