A homegrown terrorist set off an explosion that ripped open buildings in the heart of Norway's government on Friday, and then went to a summer camp dressed as a police officer and gunned down youths as they ran and even swam for their lives, police said on Friday.

The attacks killed at least 16 people in this peaceful nation's worst violence since World War II.

A police official said the 32-year-old ethnic Norwegian suspect arrested at the camp on Utoya island appears to have acted alone in both attacks, and that "it seems like that this is not linked to any international terrorist organizations at all."

The official spoke on condition of anonymity because that information had not been officially released by Norway's police.

"It seems it's not Islamic-terror related," the official said. "This seems like a madman's work."

The official said the attack “is probably more Norway's Oklahoma City than it is Norway's World Trade Center.” Domestic terrorists carried out the 1995 attack on a federal building in Oklahoma City, while foreign terrorists were responsible for the Sept. 11, 2001 attacks.

The official added, however, "it's still just hours since the incident happened. And the investigation is going on with all available resources."

Eyewitness narrates

At the youth camp, where the prime minister had been scheduled to speak Saturday, a 15-year-old camper named Elise said she heard gunshots, but then saw a police officer and thought she was safe. Then he started shooting people right before her eyes.

"I saw many dead people," said Elise, whose father, Vidar Myhre, didn't want her to disclose her last name. "He first shot people on the island. Afterward he started shooting people in the water."

Elise said she hid behind the same rock that the killer was standing on. "I could hear his breathing from the top of the rock," she said.

‘Unexploded device at shooting site’

The police official who spoke on condition of anonymity said there was at least one unexploded device at the youth camp, and that a police bomb disposal team was working on disarming it with support from military experts.

The shootings occurred after the bombing in Oslo, Norway's capital and the city where the Nobel Peace Prize is awarded.

The Oslo blast left a square covered in twisted metal, shattered glass and documents expelled from surrounding buildings in a dust-fogged scene that reminded one visitor from New York of Sept. 11.

Ian Dutton, who was in a nearby hotel, said people ‘just covered in rubble’ were walking through ‘a fog of debris.’

"It wasn't any sort of a panic," he said, "It was really just people in disbelief and shock, especially in a such as safe and open country as Norway. You don't even think something like that is possible."

Death toll could go up

Police said seven people died in the Oslo blast, and another 9 or 10 people were killed at the camp, which was organized by the youth wing of the ruling Labor Party.

Rescuers were to search to blast wreckage through the night for more victims, and Prime Minister Jens Stoltenberg said police fear there could be more victims at the camp as well.

Acting national Police Chief Sveinung Sponheim said a man was arrested in the shooting, and the suspect had been observed in Oslo before the explosion there. Police did not immediately say how much time elapsed between the bombing and the attack at Utoya, about 20 miles (35 kilometers) northwest, but reports of the shooting began appearing on Twitter about two and a half hours after the bombing.

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. Behind them, people who stripped down to their underwear swam away from the island toward shore, some using flotation devices.

Sponheim said police were still trying to get an overview of the camp shooting and could not say whether there was more than one shooter. He said several people were injured but he could not comment on their conditions.

Sponheim would not give any details about the identity or nationality of the suspect, who was being interrogated by police.

In Oslo, most of the windows in the 20-floor high-rise where Stoltenberg and his administration work were shattered. Other buildings damaged house government offices and the headquarters of some of Norway's leading newspapers.

Stoltenberg said "we don't want to speculate" on whether a terror group is responsible, and said some groups may take responsibility "to appear to be more important than they are."

Deadliest terror since London bombings

The attacks formed the deadliest day of terror in Western Europe since the 2005 London bombings, which killed 52 people.

Police said the Oslo explosion occurred at 3:30 p.m. (1330 GMT) and was caused by "one or more" bombs.

Meanwhile, police blocked off roads leading to the lake that surrounds Utoya island where the shooting took place.