The powder was discovered Friday morning as staff opened letters at the office, newspaper Aftonbladet reports. Both the police and the Swedish Security Service have been notified and so far have decided not to vacate the premises.
According to Albert Jonsson, head of the office's registrar, the sender of the letter is already known to the ombudsman and has made threats before.
"It's an individual who is known for sending threats to all sorts. We have received threats from this person before, and as far as we know, the person is in a psychiatric institution."
In other news, a woman in her 60s from Svenljunga, southwest Sweden, has been remanded into custody for allegedly sending an unknown powder to the municipal office, the local police station and the district court in nearby Borås. The woman denies the allegations.
The powder arrived on Monday and prompted the evacuation of all three building, but tests later showed the powder to be harmless.
There is no evidence linking the cases in Borås and Stockholm at this time.