Introducing Steinar as son of a Norwegian father with a Swedish mother, and therefore has dual citizenship. You can't have dual citizenship in Norway as you may have in Sweden. So the Swedish research team have missed this fact. Steinar could only have had Norwegian citizenship when working for the Norwegian police in Oslo. And you also need to be a Swedish citizen to be allowed to work for the Swedish police.
In the flashback, where Inez tells about the murder and the shadowing of her husband, she does not wear glasses, and the glasses is the clue, that leads Beck and Steiner to the solution.