Who Killed My Sister?

As I returned from a short family vacation, I was struck to find that my beloved sister had passed away. The only thing that she left behind were some vague messages to me tagged as her last wish. Amongst them, a bizarre string of letters caught my eye, "A・ C・ F・ ?・O". Despite the tumultuous emotions that weighed heavily on my mind, I was determined to decipher the hidden meanings to uncover the truth. The journey to uncover the truth was not an easy one. With each riddle that I managed to solve, I was only brought closer to the culprit. However, as I started to delve deeper into the mystery, the suspect's account was not in synchronization with the pieces of evidence. Are you willing to take up the challenge and uncover the truth alongside me? Put on your thinking caps and solve the riddles that would guide us through this web of secrets. Can we find the real killer and bring them to justice? The clock is ticking, and we have a daunting journey ahead of us.