3 ideas
In a tight social circle, the victim's power over others can be the motive. Jukka is portrayed as someone who knew and used his friends' private vulnerabilities. That spreads plausible motive across the whole group instead of pointing at one obvious suspect. The investigation therefore becomes a mapping of who had something to hide, not who had a grudge.
Because Maria shares a background with the suspects, she reads their codes, evasions and relationships faster than an outsider could. The same closeness also clouds her judgment. Her attraction to Antti Sarkela, one of the people present, compromises her neutrality. Insider knowledge speeds up understanding while making impartiality harder to keep.
Maria's authority is repeatedly discounted by male colleagues and by witnesses who treat her as too young, too female, or too temporary to count. The novel shows her having to prove legitimacy in every interaction, and competence is not assumed by default. Reading institutions through the newcomer who doesn't fit the expected profile exposes norms that insiders take for granted.