How body armor became a common feature in mass shootings.

Doing a quick google search, I came across this article from MSN.com that talks about studies being done by a nonpartisan organization called Violence Project that particular studies gun violence.  Their studies found that within the past 4 decades (40 years), 21 of the mass shooters were wearing some form of body armor protection.  Further into the article, Aaron Westrick who is a body armor expert and criminal justice professor states, “Obviously if you take the time to put on a tactical vest … it tells you that they know they’re going to war."
We are potentially talking about from the Colombine mass shooting, Sandy Hook, Sutherland Springs, and then some.