Prosecutor Alf Johansson told news agency TT that there are no suspects at this point.
"First, I will investigate whether a crime has been committed, and in connection with that, too, if there is someone who can be justifiably suspected," he said to TT.
Wallström wrote that she welcomed the investigation. "I have nothing to hide, and it is good that this will be cleared up. I'm continuing to do my job," she wrote, according to TT.
The labor union, which owns the apartment in question, also wrote in a statement that they welcomed the preliminary investigation and that they would support the prosecutor's work fully.