Along with his co-author Sara Tomek, Adam Lankford examined the wide-spread belief that mass killings are socially contagious for up to two weeks in a recent article in the journal Suicide and Life Threatening Behavior. Their study compared observed chronological clusters of mass killings in the U.S. between 2006 and 2013 and clusters of in 500 hundred simulations containing 116,000 randomly generated dates. While they did not find any evidence of short term contagion, they allow the possibility of a longer term copycat effect.
The complete text of their article is available through the below link.
http://onlinelibrary.wiley.com/doi/10.1111/sltb.12366/full