The FBI defines active shooter incidents as those in which an individual actively kills or attempts to kill people in a populated, public area. But it does not include shootings that are deemed related to other criminal activity, such as robbery or fighting over drug turf. Active shootings may involve just one shot being fired at just one target, even if the target isn’t hit.

To compile its list, the FBI hired academics at the Advanced Law Enforcement Rapid Response Training Center at Texas State University. Police departments don’t collect data, so the researchers had to find news stories about these incidents.

It isn’t surprising that people will miss cases or occasionally misidentify them when using news stories, but the FBI was unwilling to fix its errors when I pointed them out. My organization, the Crime Prevention Research Center, has found many more missed cases and is keeping an updated list. Back in 2015, I published a list of missed cases in a criminology publication.