The moment the call comes in: responding to an active shooter

Ambulances and police gather at Erlanger in Chattanooga.

Even for the men and women who work in the back of ambulances, the men and women who constantly straddle the line between life and death, the call of an "active shooter" is different.

The adrenaline pumps harder. There are more victims, more ambulances, more fear. The call demands a higher level of focus.

On a normal shooting call, Hamilton County Emergency Medical Services staff typically holds the perimeter, out of harm's way, until law enforcement can give the all-clear.

But there's no time for that during an active shooter call.

Paramedics and EMTs strap on bulletproof vests and get closer - sooner - than they normally would.

That's what happened when the active shooter call came in at the U.S.