Former FBI Profiler Says Everyone Needs to Look for Mass Shooter Warning Signs

A former FBI profiler says the public needs to take a more active role in identifying and reporting signs of potential mass shooters to the proper officials ... in light of devastating attacks like the one at a Catholic school in Minnesota Wednesday.

We spoke with Mary Ellen O'Toole -- a former profiler and author who wrote a prominent report on the Columbine High School shooting back in the day -- on "TMZ Live" ... and, she told us it matters to determine motive because these shooters aren't just "crazy."

O'Toole says past behavior plays a huge role in explaining why these shootings happen ... and the reasons are never as simple as good versus evil.

The forensic behavioral consultant says the FBI and Secret Service regularly produce reports all about the motives of people who commit heinous crimes, such as mass shootings ... because they're trying to inform the public of warning signs and the danger they pose.

O'Toole lays out the three reasons she thinks people ignore the signs when it comes to troubling behavior from their loved ones ... explaining people consistently try to normalize the issues they see instead of bringing in the authorities or help from mental health professionals.

However, O'Toole makes it clear law enforcement can't be around 24/7 ... and, the first line of defense against mass shootings -- like Wednesday's, which resulted in two kids killed and as many as 17 people injured at the Annunciation Catholic School in Minneapolis -- is made up of friends and family.

Of course, the suspected shooter in this incident -- Robin Westman -- reportedly left behind a manifesto which might give more insight to motive.

Bottom line ... Mary says people need to be much more proactive about their loved ones' behavior and potential personality changes.