"In not a single case was the killing stopped by a civilian using a gun."

[JDPriestly]

67. If you read the Mother Jones article, you learn that it adopts the FBI definition of a mass shooting

Generally, there are three terms you'll see to describe a perpetrator of this type of gun violence: mass murderer, spree killer, or serial killer. An FBI crime classification report from 2005 identifies an individual as a mass murderer if he kills four or more people in a single incident (not including himself), typically in a single location. (The baseline of four fatalities is key—more on this just below.)

The primary distinction between a mass murderer and a spree killer is that the latter strikes in multiple locations, though still in a relatively short time frame. A serial killer is distinguished by striking over a longer time frame, in multiple locations, with opportunity for what the FBI report refers to as "cooling-off periods" in between attacks.

How often do mass shootings occur?
In July, in the wake of the movie theater slaughter in Aurora, Colorado, we analyzed and mapped 60 mass shootings from the last 30 years. As we delved into the research, we realized that robust data on this subject was hard to come by, in part due to the lack of clear criteria. We were focused on the question of how many times Aurora-like events—which seemed to be chillingly frequent—had actually happened, and we honed our criteria accordingly:

We excluded crimes involving armed robbery or gang violence;
The event had to have occurred in essentially a single incident, in a public place;
And the killer, in accordance with the FBI report, had to have taken the lives of at least four people.

http://www.motherjones.com/mojo/2012/08/what-is-a-mass-shooting

The article to which the OP links links to this explanation of the criteria used in the article to define a mass murderer.

It does not include people who injure several people and only kill three.

Clearly, if the killer is stopped before he succeeds in killing four people, the 2005 FBI criteria would exclude him.