By Eric Levenson and Sheena Jones, CNN

(CNN) -- A preliminary autopsy on Mollie Tibbetts' body determined that her death was a homicide resulting from multiple sharp force injuries, the Iowa State Medical Examiner said.

Tibbetts, a 20-year-old student at the University of Iowa, went missing after an evening jog on July 18, sparking a major search in the region. A body that officials believed to be hers was found on Tuesday in a rural area, hidden under corn stalks, and the preliminary autopsy confirmed her identity.

Cristhian Bahena Rivera, a 24-year-old Mexican man who illegally came to the United States, was charged with her murder on Wednesday after he confessed to following her on her run. In an arrest affidavit, Rivera said he remembered getting mad at her; what happened afterward was "blocked" from his memory. When he came to, he realized he had put a bleeding Tibbetts in his trunk.