Police said they have not yet established if a Norwegian man charged with two attacks that claimed 92 lives Friday had acted alone or not, dpa reported.
The shooting rampage at Utoya island where 85 people were killed lasted one and half hours, Oslo police chief Sveinung Sponheim told reporters.
Police were continuing to investigate witness accounts claiming that a second gunman was at the scene of a shooting attack on a youth camp on Utoya island near Oslo, Sponheim said.
The 32-year-old surrendered when police forces contacted him on the island.
Four or five people were still reported missing at the island, suggesting the death toll could rise. Police were continuing to search the waters off the island, he said.