Mollie Tibbetts died from stab wounds.  A medical examiner announced Thursday that preliminary autopsy results from the state medical examiner’s office show the 20-year-old Brooklyn woman was the victim of a homicide who died from “multiple sharp force injuries.”  State medical examiner Dennis Klein said in an interview that the finding means a sharp-edged or pointed object such as a knife was used to attack Tibbetts. He declined comment on the details of her injuries, and said that his office would hire consultants, including forensic anthropologists, to analyze her remains further and make additional findings.  Tibbetts had disappeared while running July 18.  Her body was found Tuesday in a cornfield outside of Brooklyn.  24-year-old Cristhian Bahena Rivera is charged with first degree murder in her death.