Though the prime minister cautioned against jumping to conclusions about the gunman's motives, both attacks were in areas connected to the left-leaning Labour Party, which leads a coalition government. The youth camp, about 20 miles (35 kilometers) northwest of Oslo, is organized by the party's youth wing, and the prime minister had been scheduled to speak there Saturday.

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Sponheim said a man was arrested in the shooting, and the suspect had been observed in Oslo before the explosion there. But he refused to confirm the suspect's identity as reported by Norwegian media.

Sponheim said the camp shooter "wore a sweater with a police sign on it. I can confirm that he wasn't a police employee and never has been."

Aerial images broadcast by Norway's TV2 showed members of a SWAT team dressed in black arriving at the island in boats and running up the dock. People who had stripped down to their underwear moved in the opposite direction, swimming away from the island toward the mainland, some using flotation devices.

The United States, European Union, NATO and the U.K., all quickly condemned the bombing, which Britain's Foreign Secretary William Hague called "horrific" and NATO Secretary-General Anders Fogh Rasmussen deemed a "heinous act."

"It's a reminder that the entire international community has a stake in preventing this kind of terror from occurring," President Barack Obama said.

Obama extended his condolences to Norway's people and offered U.S. assistance with the investigation. He said he remembered how warmly Norwegians treated him in Oslo when he accepted the Nobel Peace Prize in 2009.

Britain's Queen Elizabeth II wrote to Norway's King Harald to offer her condolences and express her shock and sadness at the shooting attacks in his country.

A U.S. counterterrorism official said the United States knew of no links to terrorist groups and early indications were the attack was domestic. The official spoke on condition of anonymity because the investigation was being handled by Norway.