Investigators are still searching the surrounding waters, where people fled the attack, which followed an explosion in nearby Oslo that killed seven.
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 killed 191.
Police official Roger Andresen told reporters that the total death toll was now 91 and that a suspect was in custody being questioned for both assaults and is cooperating with the investigators.
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.
"He is clear on the point that he wants to explain himself," Roger Andresen told reporters Saturday.
National police chief Sveinung Sponheim told NRK that the suspected gunman's Internet postings "suggest that he has some political traits directed toward the right, and anti-Muslim views, but whether that was a motivation for the actual act remains to be seen."
Andersen said the suspect posted on websites with Christian fundamentalist tendencies. He did not describe the websites in any more details.
A police official said the suspect appears to have acted alone in both attacks, and that "it seems like 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."
Norway has not changed its threat level after twin attacks on the capital and a nearby island retreat, the justice minister said Saturday.
Justice Minister Knut Storberget told reporters Saturday the government was in constant discussion with police and were continually assessing it.
"The debate on the threat level is ongoing," Storberget said.
Prime Minister Jens Stoltenberg told reporters Saturday that he had spent many summers on the island of Utoya, which was hosting a youth retreat for his party.
Utoya is "my childhood paradise that yesterday was transformed into Hell," he said at a news conference in the capital at which Storberget also appeared.
Johan Fredriksen, another police official, said Saturday a SWAT team was put on standby after a bombing in Oslo.
When asked how long it took the SWAT team to arrive at the island after the shooting began, Fredriksen said: "It takes the time it takes to drive fast." He said that was about 30 minutes.
Police initially said about 10 were killed at the forested camp on the island, 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.
Maeland said the death toll could rise even more. He said others were severely wounded, but police didn't know how many were hurt.
The island is about 500 yards (meters) from one shore of Tyrifjorden lake, an oddly shaped body of water that is 15 miles (25 kilometers) at its longest and 8 miles (12 kilometers) at its widest.