The man in custody allegedly opened fire at the summer youth camp run by the Labour Party, the political party of both Stoltenberg and Storberget, just hours after explosions ripped through a government building holding Stoltenberg's office in the capital city of Oslo. According to media reports, the suspect had been seen in Oslo earlier in the day. Oslo is 45 minutes from the island of Utoya.
Explosives reportedly were found on the island after the assault.
Twenty ambulances and four air ambulances were dispatched to the island to pick up the wounded. Some of the campers at the island jumped into the water to attempt to swim to safety.
Stoltenberg was uninjured in the Oslo bombing. He was scheduled to visit the youth camp today.
Josh Haskell is a Carnegie Fellow.
ABC News' Miguel Marquez in Oslo, Norway, and Michael S. James contributed to this report.