A gunman dressed in a police uniform opened fire at the political youth camp at Utoya. The camp is for young members, between the ages of 14 and 18, of the prime minister's Labor Party.
SWAT teams were captured on camera running toward the camp, as campers were seen swimming away from the island.
Hours before, a powerful bomb blast ripped apart government buildings in Norway's capital. The bombing is the deadliest in Oslo since World War II. The explosion tore open several buildings, including the prime minister's office. Prime Minister Yens Stoltenburg was not hurt.
A 32-year-old Norwegian man has been arrested for both attacks. A Norwegian police official said he does not appear to be linked to Islamist terrorism. It's not clear whether he was acting alone.
U.S. government sources told ABC News there may have been multiple explosions and at least one of them was the result of a massive vehicle bomb. Windows were shattered at the 20-floor high-rise government headquarters building.
A Norwegian government official says there are people trapped in some of the buildings. A senior U.S. diplomat in Oslo said they believe all personnel in the country are safe and accounted for.
According to Norwegian TV, an unknown group called Helpers of the Global Jihad posted a message that said this was only the beginning.
Stoltenberg urged Norwegians to not cave in to fear after the attacks.
"It's frightening. That's not how we want things in our country. But it's important that we don't let ourselves be scared. Because the purpose of that kind of violence is to create fear," he told Norwegian broadcaster NRK.
The attack comes as Norway grapples with a homegrown terror plot linked to al Qaeda. Last week, a Norwegian prosecutor filed terror charges against an Iraqi-born cleric for threatening Norwegian politicians with death. It is unknown if it's connected with this attack.
The Associated Press contributed to this story.