What prompts Nyström to check on his neighbors in the novel, Faceless Killers?

Nyström, Johannes and Maria Lövgren's neighbor, wakes in the middle of the night because he doesn't hear his neighbor's horse whinnying as usual. He thinks he sees that his neighbors' window is open, which is unusual, and he hears someone calling for help. Going over to investigate, he finds that the window has been broken and that Johannes has been killed. Maria has been strangled and is barely alive.