The armed citizen is the first responder.
But wait,
The NYT assures us that Not a Single Instance of Self-Defense With Guns...
NYT Amnesia: Not a Single Instance of Self-Defense With Guns
http://www.freerepublic.com/focus/f-news/3602299/posts
The monthly magazine by the NRA has a regular feature on this.
There are 18 more instances listed at the link below, (27 altogether, but 9 are duplicates from this list).
http://gunwatch.blogspot.com/2015/05/mass-killings-stopped-by-armed-citizens.html
How often do unarmed citizens stop massacres?
February 19, 2014, Detroit, MI - a mother fires a Hi-Point carbine at intruders who had broken into her house, driving them off.
The list is not exhaustive.
But,but, but........ Those are gun free zones