Mr Andresen added that the suspect was co-operating with police.
He said: "He is clear on the point that he wants to explain himself."
At least 84 people were killed when a gunman opened fire on an island outside Oslo. That followed a bombing in the capital that killed seven. One man is suspected in both.
The killer, dressed as a police officer, gunned down at least 84 people on the island before being arrested.
Investigators are still searching the surrounding waters, where people fled the attack, which followed an the explosion in central Oslo.
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.
Mr Andresen told reporters that the total death toll was now 91.
Though police did not release the suspect's 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.
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".
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 organisations at all".
"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 the twin attacks, the justice minister said Saturday.
Justice Minister Knut Storberget told reporters that the government was in constant discussion with police and were continually assessing it.
"The debate on the threat level is ongoing," Mr Storberget said.
Prime Minister Jens Stoltenberg told reporters 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 Mr Storberget also appeared.
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 they had discovered many more victims.
Mr 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 from one shore of Tyrifjorden lake, an oddly shaped body of water that is 15 miles at its longest and 8 miles at its widest.