An Oslo court has ruled that confessed Norwegian mass murderer Anders Behring Breivik must remain in pretrial detention for another eight weeks, including four weeks in solitary confinement.

Judge Anne Margrethe Lund announced the decision after a closed-door hearing Monday attended by the 32-year-old right-wing extremist. The French news agency says the judge did not find the isolated detention unreasonable due to the risk of Breivik contacting possible accomplices through other inmates and tainting evidence.

Prosecutors said the additional eight weeks are needed to continue preparing formal charges against Breivik, who has confessed to the July 22 massacre that left 77 people dead.

Initially the district court ordered Monday's hearing be open to the public, but an appeals court reversed the decision. The courtroom was free of survivors, victims' family members, the media and the general public.

With the ruling, Breivik will remain in custody until November 14, when a new detention hearing will be held. He will remain in solitary confinement for half that time.

Police believe Breivik acted alone when he detonated a bomb outside government offices in Oslo and went on a shooting rampage on Utoeya island, 40 kilometers northwest of the capital. But officials are still considering the possibility Breivik may have had some help.