Noting that the details of the 911 call and the forced entry were only made public after media pressure, Shepard also said that nothing has been said about DNA collected at the crime scene.
The Isthmus story stirred outrage in Madison, where residents remain on edge. The Wisconsin State Journal hammered the Dane County dispatch in an editorial today titled "No apology? You better find one." "Apologize from the heart," the editorial began. "Find out what went wrong and fix it."
Since Zimmermann's death, her family has created a scholarship in her name. Zimmermann, a Wisconsin native and junior at the university, was a medical microbiology and immunology major who hoped to attend medical school.
Police quickly ruled out Zimmermann's fiance, who discovered her body and reported it to 911, as a suspect in the case. The investigation has focused in part on Madison's homeless and transient communities, with tips coming in from "residents who saw people going door-to-door asking for money on the day of the homicide."
Some of those people were interviewed and arrested on other charges, Madison police confirmed, but no one has been named a suspect in the murder.
ABC News' Emily Friedman contributed to this report.