It turned out that Michael was responsible for Karen's death.
According to the Sheriff, At first, When they arrested Michael he tried to deny everything. But after seeing that the cops did not believe him he then confessed that he went to Karen's house on the day of her murder but he still insisted that he didn't kill her, he said when he got to her house he was about to propose to her but on getting there he realized that she looked "somehow off". He thought she was sick so he asked her if she was okay but she said she was fine but he didn't believe her, so he then decided to push back the proposal. According to him when he left her she was okay. He said he even helped her to cook and clean. He said he then told her that he would come back later to check on her, then he left. But as he was going he realized that he left his office keys on her kitchen counter so he went back.