There is some confusion about Missy's death. What is factual is that she was both shot and she was also beaten and stabbed with whatever object the killer was carrying (I don't think they have ever released what the killer used, a lot think it was a hammer or screwdriver). IIRC, she was stabbed in the face and chest, then shot in the head.
The confusion of Missy's cause of death stems from when her death was first reported, it was said she died from puncture wounds. But now it appears to be that she died by gun shot after the FBI released their homicide statistics, which stated one homicide death by gunshot in the county that Missy lived in. There was only one death in that county at the time, which was Missy, so everyone is now assuming that her death was caused by the gun shot wounds rather than the stabbings.