Shattered glass could be seen in the snow outside the building’s entrance, as well as cracks in a thick glass door.
Overhead lamps dangled from wiring, and there were black marks on the ground at the foot of the door, presumably from the blast.
“One of the hypotheses is that it is an act of terrorism,” Frode Larsen, the head of the police’s joint unit for investigation and intelligence, told public broadcaster NRK.
“But we are not completely stuck on that. We have to be open to the possibility that there may be other causes behind what has happened,” he said, speaking on the sidelines of a news conference.
Police were searching for the perpetrators but had “no suspects” yet, Larsen told journalists.
Oslo police official Grete Lien Metlid appeared to confirm that an explosive had been thrown at the embassy.
Asked by a reporter if the device used was a hand grenade, she replied: “We haven’t said anything about what was thrown”.
Link to Middle East war?
US embassies have been placed on high alert in the Middle East over American military operations in Iran and several have faced attacks as Tehran hits back at industrial and diplomatic targets.
Investigators in Oslo have not ruled out a possible link to the war in the Middle East.
“It is natural to see this in connection with the current security policy situation,” Larsen said, adding that police had increased security at the scene after the attack.
Prime Minister Store said security had also been beefed up at “at other American, Israeli, and also Jewish targets” in Norway, but stressed that “nothing indicates that the situation is dangerous for people in Oslo or elsewhere in the country”.
The Norwegian security service PST told AFP it had called in extra staff to assist police with the investigation.
Spokesman Martin Bernsen stressed there had been “no change” to the threat assessment level in the Scandinavian country, which has been at three on a five-point scale since November 2024.
He did not say if any threats had been made against US interests in Norway prior to the explosion.
Investigators examined the scene overnight, while dogs, drones, and helicopters were brought in to search for the perpetrators, Oslo police said in a statement.
Security is normally high outside US embassies worldwide. It was not immediately known what security the Oslo embassy had at the time of the incident.
The US embassy in neighbouring Sweden issued a security alert after the Oslo incident, urging Americans to “exercise caution around the vicinity of the US embassy” due to sudden demonstrations, and to “keep a low profile”.
A 16-year-old identified only as Edvard told TV2 that he had been watching television when he heard the blast in Oslo.
“My mother and I first thought it came from our house so we looked around a little, but then we saw the flashing lights outside the window and a ton of police,” he said.
“There were police dogs and drones and police with automatic weapons and helicopters in the air,” he said.
-Agence France-Presse