A judge ordered eight weeks detention on Monday for the Norwegian man who has admitted a bombing and shooting massacre that killed about 90 people and who claimed in court to have two more groups of collaborators.
Custody, in line with prosecutors' request, will allow them to investigate the case against Anders Behring Breivik, 32, an anti-Islamic zealot who had previously claimed sole responsibility for Friday's attacks. The custody can be extended.
Judge Kim Heger said that Breivik would be locked up alone with no incoming letters, media nor visitors, except for his lawyer. A trial could be a year away.
"The accused has made statements today that require further investigation, including that 'there are two more cells in our organisation," Heger told a news conference.
Breivik told the court that he acted to save Europe from Islam.
He had previously said he had acted alone.