OSLO, Norway (AP) — Terrorism ravaged long-peaceful Norway on Friday when a bomb ripped open buildings including the prime minister’s office and a man dressed as a police officer opened fire at a nearby island youth camp. At least seven people were killed and 15 injured in the bomb blast, the nation’s worst attack since World War II.

Police Inspector Bjoern Erik Sem-Jacobsen said a suspect in the shooting has been arrested. He said the gunman, who was dressed as a police officer, pulled out a gun and started firing into the crowd of youths. There were unconfirmed reports that five people were wounded in that shooting.

Acting Police Chief Sveinung Sponheim told broadcaster NRK that investigators suspect the two attacks are linked.

A square in Oslo, where the Nobel Peace Prize is awarded, was covered in twisted metal, shattered glass and documents expelled from surrounding buildings, which house government offices and the headquarters of some of Norway’s leading newspapers. Most of the windows in the 20-floor high-rise where Prime Minister Jens Stoltenberg and his administration work were shattered.

Stoltenberg was working at home Friday and was unharmed, according to senior adviser Oivind Ostang.

Public broadcaster NRK showed video of a blackened car lying on its side amid the debris. An AP reporter who was in the office of Norwegian news agency NTB said the building shook from the blast and all employees were evacuated. Down in the street, he saw one person with a bleeding leg being led away from the area.

Oslo police said the explosion was caused by “one or more” bombs, but declined to speculate on who was behind the attack. They later sealed off the nearby offices of broadcaster TV 2 after discovering a suspicious package.

At Utoya, an island outside Oslo, a gunman dressed in a police uniform opened fire at a Labor Party youth camp, shooting several youths, party spokesman Per Gunnar Dahl told The Associated Press.

“There has been an incident where a man dressed in a police uniform started shooting among the youngsters on the island. This created a panic situation where people started to swim from the island” to escape, he said. Dahl said unconfirmed reports said five people were hit.

It wasn’t immediately clear if the two attacks were related.

The explosion occurred at 3:30 p.m. (1330 GMT), as Ole Tommy Pedersen stood at a bus stop 100 meters (yards) away.

“I saw three or four injured people being carried out of the building a few minutes later,” Pedersen told AP.

The United States quickly condemned the bombing, and the U.S. Embassy in Norway warned Americans to avoid downtown Oslo.

In Washington, State Department spokeswoman Heide Bronke Fulton called the violence “despicable.” There has been no confirmation of any U.S. casualties, she said.

The U.S. has offered help to Norwegian authorities but there has been no specific request for assistance, she said.

The attacks come as Norway grapples with a homegrown terror plot linked to al-Qaida. Two suspects are in jail awaiting charges.

Last week, a Norwegian prosecutor filed terror charges against an Iraqi-born cleric for threatening Norwegian politicians with death if he is deported from the Scandinavian country. The indictment centered on statements that Mullah Krekar — the founder of the Kurdish Islamist group Ansar al-Islam — made to various news media, including American network NBC.

Terrorism has also been a concern in neighboring Denmark since an uproar over cartoons of the Prophet Muhammad six years ago. Danish authorities say they have foiled several terror plots linked to the 2005 newspaper cartoons that triggered protests in Muslim countries. Last month, a Danish appeals court on Wednesday sentenced a Somali man to 10 years in prison for breaking into the home of the cartoonist.