Norwegian police believe the man who killed 76 people in twin attacks last Friday probably acted alone.
At his first court appearance, Anders Behring Breivik had claimed two more cells were working with him but police are sceptical.
However, with criticism growing over how long it took to apprehend Breivik, the police are ruling out nothing.
Acting Oslo police chief Sveinung Sponheim told reporters at a press conference: “We don’t have any concrete suspects, but we are investigating on it of course, checking this person in or out, if he exists we don’t know yet. But this will be an important task with the investigation.”
More than 100,000 Norwegians held a rally on Monday evening, many carrying white and red roses to mourn the dead and to show their unity against violence.
Norwegian citizen Karl Emil Lindgren said: “It is just too much to take in, how bad it is, but it is beautiful to see….. people come and showing their support to the lost ones …”
Crown Princess Mette Marit, who lost a family member in the massacre on Utoeya island, was among the mourners. Her step-brother, who worked as a security guard there, died protecting his 10- year-old son and other children.
Monday’s mass rally was probably the biggest to be held in Oslo since the end of World War II.