"It is a person who has handed in an object that we suspect could be dangerous," police spokesman Magnus Jansson Klarin told TT earlier in the day.
The police evacuated parts of the building and cordoned it off. After the national bomb disposal unit assessed the object, the cordons were lifted and personnel were able to return.
It turned out to be old dynamite, which a police patrol has now taken away to be transported elsewhere, the police write on their website.
The alarm about the object was raised at 4 p.m. on Monday, and the cordons were lifted less than an hour later.
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