Who Killed The Couples?

Riddle: George and his wife were found dead in a building, the detective on arrival found a bullet covered in blood in the room where the crime had been committed. The detective in charge of the case ordered his assistant to ask the maid, the security man, and the cook some questions that might help fish out the culprit. The assistant asked the maid where she was on the day this couple was killed, the maid replied that she wasn't around when the incident happened. She continued that she went to get groceries from the shop next door. The police asked the security man that why hadn't he heard any gunshot, the security man replied that he slept off as he was tired that day so he hadn't heard the gunshot. The assistant asked the cook what had happened on that day and she replied that she was cooking breakfast when she heard a noise in the couple's room. She continued that by the time she got to the room she saw the couples covered in their own pool of blood and she had immediately called the police nearby for help. The detective immediately knew who the culprit was. How did he know?