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.