Police in Norway says the suspect in Friday's twin attacks that killed at least 92 people has told them he acted alone.
However, police say that they must verify everything that he said.
"Some of the witness statements from the island have made us unsure of whether there was one or more shooters," acting police chief Sveinung Sponheim told a news conference.
The police chief added that Anders Behring Breivik had admitted to the facts of both the bombing and the shooting but had not admitted criminal guilt.
Another 97 people were wounded in the attacks - the worst massacre committed in Norway since World War Two.
Several people also remain missing, including a young women with dual New Zealand/Norwegian citizenship.
Anders Behring Breivik, 32, is charged with committing acts of terrorism.
He was arrested after the massacre on the island of Utoeya in a lake 40km from Oslo.
Lawyer Geir Lippestad says his client is willing to explain himself in a court hearing on Monday.
"He thought it was gruesome having to commit these acts, but in his head they were necessary," Mr Lippestad told Norwegian media.