Bomb technicians, police, and the emergency services were all called to the offices in the city's harbour district after police found an envelope filled with white powder.
“A written threat has also been sent to the company,” Anders Engström from the local police told Swedish state broadcaster SVT.
Police waited until 10am on Sunday before reporting the finding of the threatening envelope on the first floor of the building.
“We made the assessment that we could wait because we wanted to get the right people here first,” Engström told the broadcaster.
Police believe that the written threat and the envelope both formed part of an extortion attempt against the company, although the details remain uncertain.
“That's one of the things that we believe will be cleared up during the investigation,” press spokesperson Evelina Olsson told the Sydsvenskan newspaper.
The Swedish brand, known in the UK for its fish fingers, frozen peas and crispy pancakes, was bought by the UK's Nomad Foods in 2013.
Peter Odemark, chief executive Nomad Foods' Scandinavian division, would not give any further details on the threat, saying that to do so risked interfering with the police's investigation.
Police would not say what the white powder was, but said late on Sunday that they had finished analyzing it and it was now safe to be and work in the building.