OSLO, Norway — A homegrown terrorist set off a deadly explosion in downtown Oslo before heading to a summer camp dressed as a police officer to commit one of the deadliest shooting sprees in history, killing at least 80 people as terrified youths ran and even swam for their lives, police said Friday.

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. Police director Oystein Maeland told reporters early Saturday 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,” Maeland said. “It goes without saying that this gives dimensions to this incident that are exceptional.”

A suspect in the shootings, and the Oslo explosion that killed seven people, was arrested. Though police did not release his name, Norwegian national broadcaster NRK identified him as 32-year-old Anders Behring Breivik and said police searched his Oslo apartment overnight. NRK and other Norwegian media posted pictures of the blond, blue-eyed Norwegian.

A police official said the suspect appears to have acted alone in both attacks, and that “it seems like that this is not linked to any international terrorist organizations at all.” The official spoke on condition of anonymity because that information had not been officially released by Norway’s police.

“It seems it’s not Islamic-terror related,” the official said. “This seems like a madman’s work.”

The official said the attack “is probably more Norway’s Oklahoma City than it is Norway’s World Trade Center.” Domestic terrorists carried out the 1995 attack on a federal building in Oklahoma City, while foreign terrorists were responsible for the Sept. 11, 2001 attacks.

The official added, however, “it’s still just hours since the incident happened. And the investigation is going on with all available resources.”

The attacks formed the deadliest day of terror in Western Europe since the 2004 Madrid train bombings, when shrapnel-filled bombs exploded, killing 191 people and wounding about 1,800.