Police in Norway say the man who has admitted carrying out a bombing and mass shooting is probably working alone, despite telling a court "two more cells" existed in his organisation.
At a closed court custody hearing in Oslo on Monday, a judge ordered that Anders Behring Breivik be detained for eight weeks, at least four in solitary confinement.
The 32-year-old planted a bomb on Friday outside the prime minister's office, then drove to the wooded island of Utoeya and shot dead 68 people at a Labour Party youth camp.
The gunman told the court his bombing and shooting rampage aimed to save Europe from a Muslim takeover.
His remarks at the closed hearing were relayed by the judge, Kim Heger, at a news conference.
Judge Heger said the accused admitted the killings but did not plead guilty to terrorism charges.
The killer has previously said he acted alone and police had earlier said they were trying to confirm this.
Police attorney Christian Hatlo told reporters on Monday he "cannot completely, and I stress completely, rule out that others were involved in what happened."
However police say privately that they think more cells are unlikely although security services are checking with their international partners about potential foreign links.
Judge Heger has ordered Anders Behring Breivik to be locked up alone with no letters, newspapers or visitors, except for his lawyer.
The detention, in line with a request from prosecutors, will allow them to investigate the case against Breivik.
Lower death toll
Police revised the death toll from Friday's attacks downwards to 76 from 93.
They said 68 people died on the island of Utoeya rather than 86, while eight people rather than seven died in the bomb blast in central Oslo.
The police have cited difficulties in gathering information at the island.
Huge anti-violence rally
At least 100,000 people attended a rally in Oslo on Monday in remembrance of the victims of the attacks.
Many at the vigil held white or red roses in memory of those killed.
Prime Minister Jens Stoltenberg addressed the evening crowd, his voice trembling with emotion.
"By taking part you are saying a resounding 'yes' to democracy," he said, calling rally a "march for democracy, a march for tolerance, a march for unity".
Rallies were being held in other cities around the nation.