Acting National Police Chief Sveinung Sponheim said, "We're not sure it's just one person ... based on statements from witnesses, we think there may be more," CNN reported.
Only one suspect has been arrested so far. "It's very difficult at this point to say whether he was acting alone or whether he was acting as part of a larger network," Sponheim said.
The suspect has been talking to police, but Sponheim said the interrogations were "difficult."
Earlier, Norwegian police said they were looking into whether there may have been a second gunman in the deadly attack on a summer youth camp near Oslo.
Scores of teenagers were killed at the camp Friday by a suspected gunman dressed in a police uniform.
Officials were questioning one man, a native Norwegian, but Norway's national news agency NTB reported Saturday witnesses on the island where the camp is located have told police two people participated in the attack, Voice of America said.
Norwegian officials said Saturday at least 92 people were killed in the shooting spree and an Oslo office bombing earlier.
State Police Director Oystein Maeland told a news conference early Saturday personnel were still scouring the island of Utoya and waters around it looking for victims from Friday's massacre.
"We still don't have a final overview over the number of injured," he said. "This is an incident of catastrophic proportions."
The bombing of Prime Minister Jens Stoltenberg's office in Oslo killed seven people early Friday.
Not long afterwards, at least one man dressed as a policeman gunned down scores of teenagers attending a Labor Party youth camp on the small island of Utoya near Oslo, killing at least 84 of them, Views and News from Norway reported.
The gunman was wearing a police uniform and used a handgun, shotgun and an automatic weapon, the report said.
An hour after the shooting started, police arrested 32-year-old Anders Behring Breivik, the report said. They later found explosives on the island, the report added.
Police searched Breivik's Oslo apartment overnight and earlier said it appears he acted alone in the bombing and shooting spree.
A police source told The New York Times the man had recently started a farm business and it was possible that was how he allegedly acquired materials to build bombs.
The BBC quoted Deputy Police Chief Roger Andresen as saying said Breivik's various online networking accounts "goes towards the right and that it is, so to speak, Christian fundamentalist."