Try to answer this riddle whatsapp challenge

Try to answer this riddle:
A suicide was reported at a 18 floor building, when seeing the position of that dead body it was very clear that it was a suicide and not a murder, and the police department contacted a detective specialist to investigate the case. The detective went to the eighth floor of the building where the suicide was reported. He opened the window, tossed a coin into the window and it fell to the bottom of the building, then he went to the seventh floor and did the same, then he repeated this till the first floor and concluded that it was a well planned murder and not a suicide. How did he find that? 😧