A fair-haired, blue-eyed Norwegian man who dressed as a police officer to gun down summer campers killed at least 84 people at an island retreat, police have said.
It took investigators several hours to realise the full horror of Friday's massacre, which followed an explosion which killed seven people in nearby Oslo. Police believe it was set off by the same suspect.
Police did not release his name, but Norwegian national broadcaster NRK said he was 32-year-old Anders Breivik and reported that officers had searched his Oslo apartment overnight.
Police said later the suspect had right-wing and anti-Muslim views, but the motive for the attacks was unclear.
The mass shootings are among the worst in history. With the blast outside the prime minister's office, they formed the deadliest day of terror in Western Europe since the 2004 Madrid train bombings which claimed 191 lives.
Police initially said about 10 were killed at the forested camp on the island of Utoya, but some survivors said they thought the toll was much higher. Early on Saturday police director Oystein Maeland said they had discovered many more victims.
"It's taken time to search the area. What we know now is that we can say that there are at least 80 killed at Utoya," Mr Maeland said. "It goes without saying that this gives dimensions to this incident that are exceptional."
Mr Maeland warned that the death toll could rise and said others were severely injured.
A police official said the suspect appeared to have acted alone in both attacks and "it seems like that this is not linked to any international terrorist organisations at all".
"This seems like a madman's work," he added.