The Neighbor Downstairs?

Madeline Majors, a 16 year old teenager, living her ordinary life came to a serious end. Nobody saw a person move in downstairs, the room beneath hers. But she did. When the police checked, there was no one. She was shocked to see that. Now, not only does she want to figure out the mystery, but she is very determined to proof everyone that what she says is true. But the question is, is she right and who is the murderer?