Complex sentence: His research has also found that hot-spot policing is effective at reducing the crime rate, and that Operation Ceasefire was associated with reductions in violence, but that this relationship may not be causal.
Split into simpler sentences: His research has also found that hot-spot policing is effective at reducing the crime rate. He has since published research showing that this program was associated with reductions in violence, but that this relationship may not be causal.