According to the letter from Victor's father, what happened to William?

At Ingolstadt, Victor and Henry receive a letter from Victor's father: William, Victor's youngest brother, has been murdered. While on an evening walk with the family, the boy disappeared; he was found dead the following morning. On the day of the murder, Elizabeth had allowed the boy to wear an antique locket bearing Caroline's picture. Upon examining the corpse, Elizabeth finds the locket gone; she swoons at the thought that William was murdered for the bauble. She comes to blame herself for his death. Victor's father implores him to come home immediately ­ his presence will help to soothe the ravaged household. Clerval expresses his deepest sympathies, and helps Victor to order the horses for his journey.