Some respected criminologists caution that the apparent rise in active shooter events, may not in fact be real.

"I'd be hesitant to declare trends based off a modest number of years and a modest amount of data," said Dr. James Alan Fox of Northeastern University, an expert in mass shootings who served on President Clinton's advisory panel on school shootings. Fox warns that researchers may have a difficult time finding older cases. "The question is in 2003, are you sure that all those cases that would today be called active shooters would have been locatable," said Fox. According to Fox's own research, the number of "mass killings" has stayed relatively flat over the last 35 years.