Possibilities, in no particular order of probability:
1. Bad/stupid/jumpy/dumb cop.
2. Crazy/drunk/stoned/stupid victim.
3. Unknown third party.
4. Aliens. Zeta Reticuli, not Tijuana.
Most likely scenario : She heard a rape being commited in the alley behind her house and called 911, then was so disturded by it that she went to intervene .....and got shot by the perpetrators. Perps had priveledges, ways and means to cover up the crime. The rape victims body is probably at the bottom of a river or on its way to a landfill by now. When cops investigate other cops , they dont handcuff them and search their vehicle like they do civilians, the rape victim was probably in the trunk getting cold with a snapped neck