A man strolls down Midtown Manhattan's Park Avenue during rush hour, and he looks like any other New Yorker.

The only difference is he's carrying an AR-15-style assault rifle in full view.

He starts shooting, and by the time it's over, four people and the shooter are dead.

He leaves behind a note referencing the NFL and the brain disease CTE.

Today, the real story is about CTE.

What it is, what causes it, and if it can really cause such a deadly outburst.

Now, before we look at the headlines, let's talk about what CTE even stands for.

It's chronic traumatic encephalopathy.

Now this is a progressive and degenerative brain disease that is linked to repeated head trauma,

including concussions and subconcussive impacts to your brain. And it

primarily affects mood and behavior and cognitive function. And you can't really know for sure

whether it's present until autopsy.

Now, can it really cause this kind of an outburst?

That's what we're going to talk about.

Here are the headlines.

New York City skyscraper shooter suicide note blamed NFL for brain disease.

New York City shooters targeted NFL headquarters claimed to have CTE.

Park Avenue gunman's note referred to the NFL and brain disease. So what happened?

And while we talk about what happened, here's what I want you to ask yourself, if you had been in front of this building on Park Avenue and you see a man

walking with an AR-15 style weapon, what would you have done? Ask yourself that. I think it's important

that we stay situationally aware, but we need to take it one step further

and ask ourselves, what would we do if our situational awareness uncovers a genuine threat?

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