22-year-old Philip Manshaus, who faces terror charges, appeared for a brief court session at which his preliminary detention was extended, Norwegian media reported.
Manshaus was initially remanded in custody in August, suspected of murder and a "terrorist act" that police say he filmed himself committing.
Previous hearings have been conducted behind closed doors, but Monday’s was not, enabling media to attend.
The press is generally not allowed to detail such hearings, but Oslo District Court lifted the ban, allowing statements by the 22-year-old to be made public.
He gave the Nazi salute before taking his place next to his lawyers, Audun Beckstrom and Unni Fries, in the courtroom, according to the newspaper’s report.
The defence lawyers declined to comment on the gesture or Manshaus’ reason for giving it.
He was allowed to read a statement in which he explained why he killed his stepsister and attacked the Al-Noor Mosque near Oslo in August.
He claimed his motive was a desire to protect “his people” and future generations and made reference to a “race-based conflict between different groups”, according to reports of the court meeting.
“He wanted to give reasons for his actions, but I can’t comment on the content (of what he said) due to the ban on discussing the minutes of the session. The press was present and can report if the ban is lifted,” Fries told the press.
Oslo Police District prosecutor Hilde Strand said the content of Manshaus’ statement was not “anything new” for police.
The 22-year-old has admitted killing his 17-year-old stepsister Johanne Zhangjia Ihle-Hansen, who was found dead in her apartment in Bærum on August 10th.
He also admits attacking the mosque but denies charges of terror.
He entered the Al-Noor mosque and opened fire before he was overpowered by a 65-year-old man, also on August 10th.
Just three worshippers were in the mosque at the time, and there were no serious injuries.
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.
His custody was extended by four weeks by the district court on Monday.
Police expect to complete their investigations in December and submit formal charges by Christmas.