A “powerful explosion” rocked a school building in the Swedish town of Karlstad Tuesday and could be heard throughout the city, emergency officials said.

“We are on site now and are trying to ascertain what has exploded,” Ola Akesson of Karlstad’s emergency services told the Expressen newspaper.

There have been no reports of casualties or injuries.

Akesson said the blast at the Tingvallagymnasiet school was a “powerful explosion” that could be heard in other parts of the city. Plumes of smoke could be seen rising from the site of the blast.

“There was a very loud bang. We thought it was a bomb,” a teacher at the school told the NWT newspaper.

Akesson told the TT news agency that authorities have searched through the building but “have not found what caused the explosion."

The Tingvallagymnasiet school is the oldest in Karlstad, dating back to the 1800s, and is attended by students that are aged 15 to 19, The Local reports.

The building where the explosion occurred and all nearby buildings are believed to have been evacuated.