An explosion at an ammunition factory in southern Sweden seriously injured at least one person Wednesday, police said.

Police said they were investigating the blast as an industrial accident. The factory in Karlsborg, some 137 miles southwest of Stockholm, was manufacturing small-caliber ammunition.

While there was "uncertainty about how the explosion happened, it probably did not occur in the manufacturing process itself, but probably in connection with maintenance work as a pure accident," a police statement said.

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Police said the injured person was a man who was alone inside the room where the explosion occurred. Officers have not entered the facility because a safety assessment must be conducted first.

Authorities said other workers were unharmed and the explosion did not cause a fire.

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"The situation for us is quite calm at the moment," Peter Brandels of the local rescue service told Swedish broadcaster SVT.

The factory is operated by defense manufacturer Nammo, which is based in Raufoss, Norway. According to the factory’s website, the Karlsborg plant employs about 130 people and makes "small caliber ammunition for use by assault rifles and other light weapons, including armor piercing and non-toxic/lead free ammunition."

In a statement to the Associated Press, Nammo spokesperson Fredrik Tangeraas confirmed there was an incident at the facility. "We are assessing the situation and supporting emergency services in their work," he added.