I had my first experience with this issue in 1978. The property was in Simi Valley, CA. The impact was in the 30% range.
Over time, the impact decreases, so, it might be 30% now (so soon after the incident), 20% in 2008, 10% in 2009, etc.
Randy Bell created the Bell Chart that demonstrates the way negative things impact property value.
www.realestatedamages.com. He also wrote a seminar and book on point.
One time incidents have about a 3-year impact, decreasing over time.
You do not have to have a murder to have a negative impact. A death in the home can impact the property too. This happened to us. My mother-in-law died on a Friday, and was not discovered until Monday. She lived in Atherton, CA.
We cleaned out the home, steam cleaned the carpets throughout, the drapes, and repainted it. It was bright and shinny clean. But REALTORS talk, and thier talk alone cost us almost 20% in value loss.
I have a case where the property flooded six times in 10 years. Our broker survey resulted in a wide range. Many said they would not take the listing at any price for fear of liabilities.
Hope this helps.