Taika's riddles! Riddle 4...(Hopefully this is a bit 更多 difficult than the last 3)

A man was found dead in his study. He was slumped over his 台, 办公桌 and a gun was in his hand. There was a cassette recorder on his desk. When the police entered the room and pressed the play button on the tape recorder they heard: "I can't go on. I have nothing to live for." Then there was the sound of a gunshot. How did the detective immediately know that the man had been murdered and it wasn't a suicide?